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any member of a unique class of infectious agents, which were originally distinguished by their smallness (hence, they were described as “filtrable” because of their ability to pass through fine ceramic filters that blocked all cells, including bacteria) and their inability to replicate outside of and without assistance of a living host cell. Because these properties are shared by certain bacteria (rickettsiae, chlamydiae), viruses are now characterized by their simple organization and their unique mode of replication. A virus consists of genetic material, which may be either DNA or RNA, and is surrounded by a protein coat and, in some viruses, by a membranous envelope.

Unlike cellular organisms, viruses do not contain all the biochemical mechanisms for their own replication; they replicate by using the biochemical mechanisms of a host cell to synthesize and assemble their separate components. (Some do contain or produce essential enzymes when there is no cellular enzyme that will serve.) When a complete virus particle (virion) comes in contact with a host cell, only the viral nucleic acid and, in some viruses, a few enzymes are injected into the host cell.

Within the host cell the genetic material of a DNA virus is replicated and transcribed into messenger RNA by host cell enzymes, and proteins coded for by viral genes are synthesized by host cell ribosomes. These are the proteins that form the capsid (protein coat); there may also be a few enzymes or regulatory proteins involved in assembling the capsid around newly synthesized viral nucleic acid, in controlling the biochemical mechanisms of the host cell, and in lysing the host cell when new virions have been assembled. Some of these may already have been present within the initial virus, and others may be coded for by the viral genome for production within the host cell.

Because host cells do not have the ability to replicate “viral RNA” but are able to transcribe messenger RNA, RNA viruses must contain enzymes to produce genetic material for new virions. For certain viruses the RNA is replicated by a viral enzyme (transcriptase) contained in the virion, or produced by the host cell using the viral RNA as a messenger. In other viruses a reverse transcriptase contained in the virion transcribes the genetic message on the viral RNA into DNA, which is then replicated by the host cell. Reverse transcriptase is actually a combination of two enzymes: a polymerase that assembles the new DNA copy and an RNase that degrades the source RNA.

In viruses that have membranes, membrane-bound viral proteins are synthesized by the host cell and move, like host cell membrane proteins, to the cell surface. When these proteins assemble to form the capsid, part of the host cell membrane is pinched off to form the envelope of the virion.

Some viruses have only a few genes coding for capsid proteins. Other more complex ones may have a few hundred genes. But no virus has the thousands of genes required by even the simplest cells. Although in general viruses “steal” their lipid envelope from the host cell, virtually all of them produce “envelope proteins” that penetrate the envelope and serve as receptors. Some envelope proteins facilitate viral entry into the cell, and others have directly pathogenic effects.

Some viruses do not produce rapid lysis of host cells, but rather remain latent for long periods in the host before the appearance of clinical symptoms. This carrier state can take any of several different forms. The term latency is used to denote the interval from infection to clinical manifestations. In the lentiviruses, it was formerly mistakenly believed that virus was inactive during this period. The true situation is that lentiviruses are rapidly replicating and spawning dozens of quasi-species until a particularly effective one overruns the ability of the host's immune system to defeat it. Other viruses, however, such as the herpesviruses, actually enter a time known as “viral latency,” when little or no replication is taking place until further replication is initiated by a specific trigger. For many years all forms of latency were thought to be identical, but now it has been discovered that there are different types with basic and important distinctions.

In viral latency, most of the host cells may be protected from infection by immune mechanisms involving antibodies to the viral particles or interferon. Cell-mediated immunity is essential, especially in dealing with infected host cells. Cytotoxic lymphocytes may also act as antigen-presenting cells to better coordinate the immune response. Containment of virus in mucosal tissues is far more complex, involving follicular dendritic cells and Langerhans cells.

Some enveloped RNA viruses can be produced in infected cells that continue growing and dividing without being killed. This probably involves some sort of intracellular regulation of viral growth. It is also possible for the DNA of some viruses to be incorporated into the host cell DNA, producing a carrier state. These are almost always retroviruses, which are called proviruses before and after integration of viral DNA into the host genome.

Few viruses produce toxins, although viral infections of bacteria can cause previously innocuous bacteria to become much more pathogenic and toxic. Other viral proteins, such as some of the human immunodeficiency virus, appear to be actively toxic, but those are the exception, not the rule.

However, viruses are highly antigenic. Mechanisms of pathologic injury to cells include cell lysis; induction of cell proliferation (as in certain warts and molluscum contagiosum); formation of giant cells, syncytia, or intracellular inclusion bodies caused by the virus; and perhaps most importantly, symptoms caused by the host's immune response, such as inflammation or the deposition of antigen-antibody complexes in tissues.

Because viral reproduction is almost completely carried out by host cell mechanisms, there are few points in the process where stopping viral reproduction will not also kill host cells. For this reason there are no chemotherapeutic agents for most viral diseases. acyclovir is an antiviral that requires viral proteins to become active. Some viral infections can be prevented by vaccination (active immunization), and others can be treated by passive immunization with immune globulin, although this has been shown to be effective against only a few dozen viruses.
Comparison of shapes and sizes of viruses.
attenuated virus one whose pathogenicity has been reduced by serial animal passage or other means.
B19 virus a species belonging to the genus Erythrovirus that binds to the erythrocyte P blood group antigen and is the cause of erythema infectiosum. In patients with hemolytic anemia or sickle cell disease it causes aplastic crisis; it can also cause acute arthritis. Fetal infection can cause hydrops fetalis and spontaneous abortion or death in utero. Persistent infection in immunocompromised patients can lead to chronic bone marrow failure. Called also human parvovirus B19.
bacterial virus one that is capable of producing transmissible lysis of bacteria; see also bacteriophage.
coryza virus rhinovirus.
Coxsackie virus coxsackievirus.
defective virus one that cannot be completely replicated or cannot form a protein coat; in some cases replication can proceed if missing gene functions are supplied by other viruses; see also helper virus.
dengue virus a flavivirus, existing as four antigenically related but distinct types (designated 1, 2, 3, and 4), that causes both the classic and hemorrhagic forms of dengue.
DNA virus a virus whose genome consists of DNA.
Ebola virus an RNA virus almost identical to the Marburg virus but serologically distinct; it causes a disease similar to that caused by the Marburg virus.
encephalomyocarditis virus an enterovirus that causes mild aseptic meningitis and encephalomyocarditis.
enteric v's enterovirus.
enteric orphan v's orphan viruses isolated from the intestinal tract of humans and other animals.
Epstein-Barr virus (EBV) a herpeslike virus that causes infectious mononucleosis and is associated with Burkitt's lymphoma and nasopharyngeal carcinoma; see also epstein-barr virus.
fixed virus a virus whose virulence and incubation period have been stabilized by serial passage and have remained fixed during further transmission, as opposed to a street virus.
helper virus one that aids in the development of a defective virus by supplying or restoring the activity of the viral gene or enabling it to form a protein coat.
hepatitis A virus (HAV) any virus of the genus Hepatovirus that causes hepatitis a. This has the most rapid onset of the hepatitis viruses affecting humans; transmission is easier than for the hepatitis B and C viruses, but infection generally does not persist. While infection with this virus alone is usually not life-threatening, coincident infection with hepatitis C virus is generally rapidly fatal.
hepatitis B virus (HBV) a species of genus Orthohepadnavirus that causes hepatitis b.
hepatitis C virus a species of genus Hepacivirus that causes hepatitis c; its latency period may last 30 years or more.
hepatitis D virus (HDV) (hepatitis delta virus) an unclassified defective RNA virus, thought of as a parasite of the hepatitis B virus and transmitted in the same manner; it requires enzymes and other assistance from HBV to replicate. This virus magnifies the pathogenicity of hepatitis B virus many times and is the etiologic agent of hepatitis d.
hepatitis E virus an enterically transmitted calicivirus that causes hepatitis e.
hepatitis G virus (HGV) a parenterally transmitted flavivirus originally isolated from a patient with chronic hepatitis; most infections are benign, and it is uncertain what role, if any, HGV plays in the etiology of liver disease.
hepatotropic virus a virus that primarily affects the liver, such as the hepatitis viruses.
herpes virus herpesvirus.
herpes simplex virus former name for any virus that causes herpes simplex, now called human herpesviruses; see herpesvirus.
human immunodeficiency virus (HIV) either of two species of lentiviruses that cause acquired immunodeficiency syndrome (AIDS). HIV-1 is found around the world and HIV-2 is found primarily in West Africa. Progression of HIV-2 infection to AIDS is generally slower and less extreme than that of HIV-1. The virus is believed to induce permanent infection and has a propensity toward a subset of T lymphocytes called the CD4 cells. The infected cells become dysfunctional and eventually the host's immune system is overwhelmed or exhausted; death ensues, usually as a result of infection. The virus is not transmitted through casual contact; the most common routes of transmission are through sexual intercourse, direct exposure to contaminated blood, and transplacental transmission from mother to fetus.
Human immunodeficiency virus: retrovirus particle. From Copstead, 1995.
human T-cell leukemia virus (human T-cell lymphotropic virus) former names for human T-lymphotropic virus.
human T-lymphotropic virus (HTLV) either of two related species of retroviruses that have an affinity for the helper cell type of T lymphocytes. HTLV-1 causes chronic infection and is associated with adult T-cell leukemia and a type of myelopathy. HTLV-2 has been isolated from an atypical variant of hairy cell leukemia and from patients with other hematological disorders, but no clear association with disease has been established.
influenza virus any of a group of orthomyxoviruses that cause influenza; there are at least three serotypes or species (A, B, and C). Serotype A viruses are subject to major antigenic changes (antigenic shifts) as well as minor gradual antigenic changes (antigenic drift) and cause widespread epidemics and pandemics. Serotypes B and C are chiefly associated with sporadic epidemics.
influenza A virus (influenza B virus) (influenza C virus) species in the genera Influenzavirus A, Influenzavirus B, and Influenzavirus C; see influenza virus.
La Crosse virus a virus of the California serogroup of the genus Bunyavirus, the etiologic agent of La Crosse encephalitis.
latent virus one that ordinarily occurs in a noninfective state and is demonstrable by indirect methods that activate it.
lytic virus one that is replicated in the host cell and causes death and lysis of the cell.
maedi/visna virus a lentivirus that is the etiologic agent of a type of pneumonia in sheep.
Marburg virus an RNA virus occurring in Africa, transmitted by insect bites, and causing marburg virus disease.
masked virus latent virus.
measles virus a paramyxovirus that is the cause of measles.
mumps virus a paramyxovirus that causes mumps and sometimes tenderness and swelling of the testes, pancreas, ovaries, or other organs.
Norwalk virus a calicivirus that is common cause of epidemics of acute gastroenteritis, with diarrhea and vomiting that last 24 to 48 hours.
oncogenic v's an epidemiologic class of viruses that are acquired by close contact or injection and cause usually persistent infection; they may induce cell transformation and malignancy.
orphan v's viruses isolated in tissue culture, but not found specifically associated with any disease.
parainfluenza virus any of various paramyxoviruses that cause upper respiratory tract disease of varying severity.
poliomyelitis virus see poliovirus.
pox virus poxvirus.
rabies virus an RNA virus of the rhabdovirus group that causes rabies.
respiratory syncytial virus (RSV) any of a genus of single-stranded paramyxoviruses; the name is derived from the type of disease produced (respiratory infection) and the microscopic appearance of the viruses in cell cultures. RSV can cause a wide variety of respiratory disorders ranging from a mild cold to serious or even fatal disease of the lung in the very young and very old. It regularly produces an outbreak of infection each winter and virtually disappears in the summer months. The most severe infections in children are in the very young, especially those who are preterm, immunologically compromised, or suffering from a congenital heart defect or preexisting lung disorder. Adults at risk for infection include parents and others who are repeatedly exposed to young children, for example, pediatric nurses and day care attendants. The course of infection tends to be milder in adults than in children and about 15 per cent of affected adults have no symptoms. In the very elderly these infections may have the same degree of seriousness and clinical manifestations as in the very young.
RNA virus a virus whose genome consists of RNA.
rubella virus a togavirus that is the etiologic agent of rubella.
satellite virus a strain of virus unable to replicate except in the presence of helper virus; considered to be deficient in coding for capsid formation.
simian-human immunodeficiency virus a chimeric, engineered virus with the envelope of human immunodeficiency virus and the cytoplasm and nucleus of simian immunodeficiency virus; it is used in animal models because it is a better mimic of HIV than SIV is.
simian immunodeficiency virus (SIV) a lentivirus closely related to human immunodeficiency virus that causes inapparent infection in African green monkeys and a disease resembling acquired immunodeficiency syndrome in macaques and chimpanzees.
slow virus any virus that remains latent for long periods in the infected host before the appearance of clinical symptoms.
smallpox virus variola virus.
street virus virus from a naturally infected animal, as opposed to a laboratory-adapted strain of the virus.
vaccinia virus a species of orthopoxvirus that does not occur in nature and has been propagated for many years only in the laboratory for use as an active vaccine against smallpox. The present virus is derived from the original one used by Jenner, obtained from the lesions of cowpox, but the origin of the original virus remains unclear.
varicella-zoster virus former name for human herpesvirus 3; see herpesvirus.
variola virus the virtually extinct orthopoxvirus that is the etiologic agent of smallpox. No natural infection has occurred since 1977, and no reservoir of the virus now exists.
West Nile virus a virus of the genus Flavivirus, the cause of West Nile encephalitis; it is transmitted by Culex mosquitoes, with wild birds serving as the reservoir. It was originally endemic in Africa, Asia, and Europe, but recently spread to North America.
wild-type virus street virus.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.


, pl.


1. Formerly, the specific agent of an infectious disease.
2. Specifically, a term for a group of infectious agents, which, with few exceptions, are capable of passing through fine filters that retain most bacteria, are usually not visible through the light microscope, lack independent metabolism, and are incapable of growth or reproduction apart from living cells. They have a prokaryotic genetic apparatus but differ sharply from bacteria in other respects. The complete particle usually contains either DNA or RNA, not both, and is usually covered by a protein shell or capsid that protects the nucleic acid. They range in size from 15 to several hundred nanometers. Classification of viruses depends on physiochemical characteristics of virions as well as on mode of transmission, host range, symptomatology, and other factors. For viruses not listed below, see the specific name. Synonym(s): filtrable virus
3. Relating to or caused by a virus, as a viral disease.
4. Obsolete usage. From before the development of bacteriology, any agent thought to cause disease, including a chemical substance such as an enzyme ("ferment") similar to snake venom; synonymous at that time with "poison."
[L. poison]
Farlex Partner Medical Dictionary © Farlex 2012


n. pl. vi·ruses
a. Any of various submicroscopic agents that infect living organisms, often causing disease, and that consist of a single or double strand of RNA or DNA surrounded by a protein coat. Unable to replicate without a host cell, viruses are typically not considered living organisms.
b. A disease caused by a virus.
2. A computer program or series of commands that can replicate itself and that spreads by inserting copies of itself into other files or programs which users later transfer to other computers. Viruses usually have a harmful effect, as in erasing all the data on a disk.
3. A harmful or destructive influence: the pernicious virus of racism.
The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.


Infectious disease A small, obligatorily intracellular agent ranging from 106 daltons–eg, Parvoviridae to 200 x 106–eg, Poxviridae; viral nucleic acid is single- or double-stranded, either DNA or RNA, and is a closed circle or opened and linear; viral nucleic acid is packaged within a protein coat–capsid composed of a few distinct types of protein; most have a helical or icosahedral symmetry; once inside the infected cell, the virus uses the host's synthetic capabilities to produce progeny virus; some viruses–eg, influenza virus, are 'studded' with external proteins–eg, hemagglutinins, neuraminidases
McGraw-Hill Concise Dictionary of Modern Medicine. © 2002 by The McGraw-Hill Companies, Inc.


, pl. viruses (vī'rŭs, -ĕz)
1. Specifically, a term for a group of infectious agents that with few exceptions are capable of passing through fine filters that retain most bacteria, are usually not visible through the light microscope, lack independent metabolism, and are incapable of growth or reproduction apart from living cells. They have a prokaryotic genetic apparatus but differ sharply from bacteria in other respects. The complete particle usually contains either DNA or RNA, not both, and is usually covered by a protein shell or capsid that protects the nucleic acid. They range in size from 15 nm up to several hundred nm. Classification of viruses depends on physiochemical characteristics of virions as well as on mode of transmission, host range, symptomatology, and other factors.
Synonym(s): ultravirus.
2. Relating to or caused by a virus, as a virus disease.
[L. poison]
Medical Dictionary for the Health Professions and Nursing © Farlex 2012


(vi'rus) [L. virus, poison]
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A pathogen composed of nucleic acid within a protein shell, which can grow and reproduce only after infecting a host cell. More than 400 types of viruses that cause a great variety of illness are known. All of them can attach to cell membranes, enter the cytoplasm, take over cellular functions, reproduce their parts, and assemble themselves into mature forms capable of infecting other cells.

Some of the most virulent diseases are caused by viruses, e.g., the hemorrhagic fever caused by Ebola virus. Viruses are also responsible for the common cold, childhood exanthems (such as chickenpox, measles, rubella), latent infections (such as herpes simplex), some cancers or lymphomas (such as Epstein-Barr virus), and diseases of all organ systems.

Although viral architecture is very complex, every virus contains at least a genome and a capsid.Most animal viruses are also surrounded by a lipid envelope, a bilayered membrane analogous to a cell membrane. The envelope may be parasitized from host cells. Its chemical components are phospholipids and glycoproteins. The lipid envelope is frequently dotted by spikes.

Viruses with lipid envelopes have a greater ability to adhere to cell membranes and to avoid destruction by the immune system. Both the capsid and envelope are antigenic. Frequent mutations change some viral antigens so that the lymphocytes are unable to create an antibody that can neutralize the original antigen and its replacement. The common influenza viruses have antigens that mutate or combine readily, requiring new vaccines with each mutation. The body's primary immune defenses against viruses are cytotoxic T lymphocytes, interferons, and, to some extent, immunoglobulins; destruction of the virus often requires destruction of the host cell.

When viruses enter a cell, they may immediately trigger a disease process or remain quiescent for years. They damage the host cell by blocking its normal protein synthesis and using its metabolic machinery for their own reproduction. New viruses are then released either by destroying their host cell or by forming small buds that break off and infect other cells. See: illustration; table


The 400 known viruses are classified in several ways: by genome core (RNA or DNA), host (animals, plants, or bacteria), method of reproduction (such as retrovirus), mode of transmission (such as enterovirus), and disease produced (such as hepatitis virus).


Antiviral drugs include such agents as acyclovir (for herpes simplex); oseltamivir and zanamivir (for influenza A); interferons (for chronic hepatitis B and C); ribavirin (for respiratory syncytial virus and chronic hepatitis C); and lamivudine (for HIV).

arbor virus

Former name for arbovirus.

attenuated virus

A virus with reduced pathogenicity as a result of treatment or repeated passage through hosts.

B virus

Cercopithecine herpesvirus 1.

bacterial virus


Banna virus

Abbreviation: BAV
The type species of the genus Seadornavirus (family Reoviridae), isolated in Asia. It is transmitted to humans by the mosquito or tick bite and is implicated in some cases of encephalitis.

Barmah Forest virus

A virus transmitted by mosquito bite that causes rash, fever, joint pain, and stiffness. It is found mostly in Australia and neighboring islands.

cercopithecine virus 1

Cercopithecine herpesvirus 1.

cowpea mosaic virus

A plant virus used in vaccine development to deliver antigens from pathogens and tumors. Because the virus does not infect animals, it is considered a safe vehicle for antigen display in humans and other species.

coxsackie virus

See: coxsackievirus

cytomegalic virus

Abbreviation: CMV

deer tick virus

A Flavivirus that is a cause of meningoencephalitis. It is transmitted to humans by the bite of infected ticks and causes disease similar to that caused by Powassan virus.

defective virus

A virus particle that, because of a lack of certain essential factors, is unable to replicate. Sometimes this can be overcome by the presence of a helper virus that provides the missing factor(s).

delta hepatitis virus

Abbreviation: HDV
See: hepatitis D

DNA virus

A virus such as the papilloma virus and the herpesviruses whose genome is deoxyribonucleic acid (DNA).

EB virus

Epstein-Barr virus.

enteric virus


enteric cytopathogenic human orphan virus

Abbreviation: echovirus
An orphan virus that was accidentally discovered in human feces and is not known to be associated with a disease. Initially, 33 echovirus serotypes were designated, but numbers 10 and 28 have been reclassified. Various serotypes have been associated with aseptic meningitis, encephalitis, acute upper respiratory infection, enteritis, pleurodynia, and myocarditis.

enteric orphan virus

See: enteric cytopathogenic human orphan virus

Epstein-Barr virus

See: Epstein-Barr virus

Eyach virus

Abbreviation: EYAV
A European tick-borne coltivirus implicated in some cases of meningitis and encephalitis.

filtrable virus

A virus causing infectious disease, so small it retains infectivity after passing through a filter of the Berkefeld type.
See: Berkefeld filter

fixed virus

A rabies virus stabilized and modified but only partially attenuated by serial passage through rabbits.

foamy virus


GB virus type C

Hepatitis G virus.

Guanarito virus

An arenavirus from the Tacaribe virus group that chronically infects rodents. It is the cause of sporadic outbreaks of Venezuelan hemorrhagic fever.

helper virus

A virus that permits a defective virus present in the same cell to replicate.
See: defective virus

hepatitis G virus

An RNA Flavivirus found in blood in about 2% of blood donors that may be transmitted by injection, drug abuse, sexual contact, transfusions, and childbirth (from mother to infant). It is remotely related to hepatitis C virus. It causes chronic viremia but does not seem to cause hepatitis or liver damage. Synonym: GB virus type C

herpes simplex virus

Abbreviation: HSV-1, HSV-2
Either of two human DNA viruses (HSV-1 and -2) that cause repeated painful vesicular eruptions on the genitals and other mucosal surfaces and on the skin. After initial contact with the skin or mucous membranes, the virus migrates along nerve fibers to sensory ganglia, where it establishes a latent infection. Under a variety of stimuli, such as sexual contact, exposure to ultraviolet light, febrile illnesses, or emotional stress, it may reappear, traveling back to the site of initial contact although the vast majority of herpes simplex infections are neither recognized nor symptomatic. The rash caused by the infection has a red base, on which small blisters cluster. Herpetic rashes on the mouth or nose are called cold sores or fever blisters. Synonym: herpesvirus hominis ;

In immunosuppressed patients, the virus can cause a widely disseminated rash. Some infections with HSV may involve the brain and meninges; these typically cause fevers, headaches, altered mental status, seizures, or coma, requiring parenteral therapy with antiviral drugs. In newborns, infection involving the internal organs also may occur. Experienced ophthalmologists should manage ocular infection with HSVs. Health care providers are at risk for herpetic whitlow (finger infections) from contact with infected mucous membranes if gloves and meticulous hand hygiene are not used.


Acyclovir and related drugs, e.g., famciclovir, valacyclovir, may be used to treat outbreaks of HSV-1 and HSV-2 and are also effective in preventing recurrences of disease.

Patient care

Standard precautions prevent spread of the virus. Prescribed antiviral agents and analgesics are administered; their use is explained to the patient, with instruction given about adverse effects to report.

The patient with HSV-1 is instructed to avoid skin-to-skin contact with uninfected individuals when lesions are present or prodromal symptoms are felt. To decrease the discomfort from oral lesions, the patient is advised to use a soft toothbrush or sponge stick, a saline- or bicarbonate-based (not alcohol-based) mouthwash, and oral anesthetics such as viscous lidocaine if necessary. He or she should eat soft foods. Use of lip balm with sunscreen reduces reactivation of oral lesions.

The patient with genital herpes should wash the hands carefully after bathroom use. He or she also should avoid sexual intercourse during the active stage of the disease and should practice safe sex. A pregnant woman must be advised of the potential risk to the infant during vaginal delivery and the use of cesarean delivery if she has an HSV outbreak when labor begins and her membranes have not ruptured. The patient with genital herpes may experience feelings of powerlessness. He requires assistance to identify coping mechanisms, strengths, and support resources; should be encouraged to voice feelings about perceived changes in sexuality and behavior; and should be provided with current information about the disease and treatment options. A referral is made for additional counseling as appropriate.


Caregivers with active oral or cutaneous lesions should avoid providing patient care.

herpes virus

See: herpesvirus
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EFFECT OF HIV ON IMMUNE SYSTEM: HIV contains several proteins: gp 120 protein around it and viral RNA and p24 protein inside. The gp 120 proteins attach to CD4+ receptors of T lymphocytes; HIV enters the cell and makes viral DNA; the enslaved host cell produces new viruses that bud, which destroy the host cell's membrane, causing cellular death and allowing the virus to leave to attack other CD4+ lymphocyte cells.
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EFFECT OF HIV ON IMMUNE SYSTEM: HIV contains several proteins: gp 120 protein around it and viral RNA and p24 protein inside. The gp 120 proteins attach to CD4+ receptors of T lymphocytes; HIV enters the cell and makes viral DNA; the enslaved host cell produces new viruses that bud, which destroy the host cell's membrane, causing cellular death and allowing the virus to leave to attack other CD4+ lymphocyte cells.

human immunodeficiency virus

Abbreviation: HIV
A retrovirus of the subfamily lentivirus that causes acquired immunodeficiency syndrome (AIDS). The most common type of HIV is HIV-1, identified in 1984. HIV-2, first discovered in West Africa in 1986, causes a loss of immune function and the subsequent development of opportunistic infections identical to those associated with HIV-1 infections. The two types developed from separate strains of simian immunodeficiency virus. In the U.S., the number of those infected with HIV-2 is very small, but blood donations are screened for both types of HIV.
See: illustration; acquired immunodeficiency syndrome

human papilloma virus

See: papillomavirus

human T-cell lymphotropic virus type I

Abbreviation: HTLV-I
A virus associated with adult T-cell leukemia.

human T-cell lymphotropic virus type II

Abbreviation: HTLV-II
A virus associated with hairy cell leukemia.

human T-cell lymphotropic virus type III

Abbreviation: HTLV-III
The former name for human immunodeficiency virus (HIV).

influenza virus

An RNA virus that infects the respiratory tract of humans (as well as birds, pigs, dogs, and horses), causing influenza. It is a roughly spherical virus. It is composed of eight segments, including a nucleoprotein (which folds its nucleic acids and helps them to be transcribed), a hemagglutinin (which helps it to enter cells), a neuraminidase (which helps it to bud out of infected cells), several transcriptases (which make copies of the viral RNA), a matrix protein (which supports the outer membrane), and several nonstructural proteins. The virus mutates frequently and causes annual disease outbreaks, some of which (pandemics) affect millions of people. It can be treated with antiviral drugs and prevented with annual vaccinations that target its frequently evolving antigens.
See: influenza

Inkoo virus

Abbreviation: INK
A European arbovirus of the California family of bunyaviruses. It is transmitted to humans by the bite of infected mosquitoes, causing fevers, encephalitis, and meningitis.

JC virus

A DNA papovavirus that causes progressive multifocal leukoencephalopathy in immunosuppressed patients. It is carried asymptomatically by a large percentage of the population.

Junin virus

An Arenavirus that chronically infects rodents. It is the cause of sporadic outbreaks of Argentine hemorrhagic fever, a potentially lethal infection usually found in South America.

Kunjin virus

A group B Flavivirus that is one of the causative agents of encephalitis in Australia.

Kyasanur Forest virus

A single-stranded RNA virus transmitted to humans by tick bite. It causes a two-stage illness: fever followed by a brief remission, and then meningoencephalitis, hemorrhagic pneumonia, and hepatic and splenic disruption.

Langat virus

Abbreviation: LGT
A tick-borne Flavivirus, responsible for encephalitis, primarily in mice. It causes mild disease in humans. Its genetic similarity to other tick-borne encephalitis viruses makes it a candidate for the development of encephalitis vaccines.

latent virus

A virus that has the ability to infect the host, initially causing little or no evidence of illness but persisting for the lifetime of the infected host; later on, a specific triggering mechanism may cause the virus to produce a clinically apparent disease. This occurs with herpes simplex virus, which remains latent in sensory ganglia and is reactivated by trauma to the skin supplied by the distal sensory nerves associated with these ganglia. After reactivation, the virus may cause localized or generalized lesions in the affected area and the central nervous system.

lytic virus

Any virus that, after infecting a cell, lyses it.

masked virus

A virus that ordinarily occurs in the host in a noninfective state but is activated and demonstrated by indirect methods.

neurotropic virus

A virus that reproduces in nerve tissue.

Nipah virus

Abbreviation: NiV
A member of the family of paramyxoviruses that can cause outbreaks of encephalitis and respiratory disease in humans. It is transmitted to humans from infected swine, e.g., in slaughterhouses.

Norwalk virus

Abbreviation: NLV
A Calicivirus that is the causative organism in over half of the reported cases of epidemic viral gastroenteropathy. It commonly causes nausea, vomiting, and diarrhea. The incubation period ranges from 18 to 72 hr. Outbreaks are usually self-limited. Intestinal signs and symptoms last for 24 to 48 hr. Treatment, if required, is supportive and directed at maintaining hydration and electrolyte balance. Synonym: Norwalk agent See: Calicivirus

Oliveros virus

An Arenavirus of the Tacaribe complex of viruses that normally infects rodents in the pampas of Argentina. It may cause a fatal hemorrhagic fever in humans.

Omsk hemorrhagic fever virus

A single-stranded RNA virus in the Flavivirus genus. It can be transmitted to people by the bite of infected ticks. Clinical symptoms of infection include hemorrhage, conjunctivitis, cough if the patient develops pneumonia, fevers, and meningoencephalitis.

oncogenic virus

Tumor virus.

O'nyong-nyong virus

An Alphavirus found in central Africa that causes epidemic fevers, joint pains, and swollen glands. The virus is transmitted to humans by mosquito bite.

orphan virus

One of several viruses that initially were not thought to be associated with human illness. This group includes the enteroviruses and rhinoviruses.

parainfluenza virus

One of a group of viruses that affects infants and young children. It causes respiratory infections that may be mild or may progress to pneumonia. Most infections are so mild as to be clinically inapparent.

Puumala virus

Abbreviation: PUUV
A member of the Hantavirus family and the causative agent of nephropathia epidemica.

Powassan virus

A rare Flavivirus, transmitted by tick bite, which occasionally causes encephalitis.

pox virus


Rauscher leukemia virus

See: Rauscher leukemia virus

reassortant virus

A virus whose genetic material has been recombined or reshuffled so that it contains new nucleic acid sequences, new antigenic structures, and new combinations of protein products.

respiratory syncytial virus

Abbreviation: RSV
A single-stranded RNA virus that is an important cause of upper and lower respiratory tract disease in infants, children, and the older population. When limited to the upper respiratory tract, RSV causes symptoms of the common cold. In the lower respiratory tract, it causes bronchiolitis, pneumonia, or respiratory distress and can be life-threatening. Respiratory syncytial virus is the most common cause of lower respiratory infections in infants and children under age 2. It is spread by physical contact, usually with infected nasal or oral secretions. In the U.S., its season begins in the fall and peaks in winter. About 90,000 young children are hospitalized with RSV infections each year in the U.S.


Three to five days following exposure to RSV, the patient typically develops an upper respiratory infection lasting 1 to 2 weeks with cough, mild to moderate nasal congestion, runny nose, and low-grade fever. If the infection spreads to the lower respiratory tract, symptoms worsen and may include wheezing and difficulty breathing. Infants and children with RSV pneumonia exhibit retractions; rapid grunting respirations, poor oxygenation, and respiratory distress. Vomiting, dehydration, and acidosis may occur.


Diagnosis is based on signs and symptoms and confirmed by isolating RSV from respiratory secretions (sputum or throat swabs). Immunofluorescence techniques, enzyme immunoassays, or rapid chromatographic immunoassays provide rapid identification of viral antigens for diagnosis.

Patient care

Treatment is mainly supportive. Antibiotics are not effective. Acetaminophen or ibuprofen are given for pain or fever. Oxygen is administered if the patient’s oxygen saturation SpO2 falls below 92%. Bronchodilators, such as albuterol and epinephrine, are used to treat wheezing. In patients with severe RSV infections, noninvasive positive-pressure ventilation or intubation and mechanic ventilation are required. Intravenous fluids are administered as prescribed if the patient cannot take enough fluid orally. Nasopharyngeal suction may be needed to clear congestion (by bulb syringe for infants).

Strict adherence to infection control measures is important in preventing an outbreak in any facility. This includes using meticulous hand hygiene (the most important step in preventing RSV spread) before donning gloves for patient care, after removing gloves, and if any potentially contaminated surfaces have been touched. Standard and contact precautions should be observed for all patients with known or suspected RSV (gown, mask and eye protection for direct contact with respiratory secretions or droplets). Protective coverings should be removed in this order: gloves (followed by hand hygiene), goggles or face shield, gown, and finally mask or respirator, discarding them in an infectious waste container in the patient’s room. The patient with RSV should be in a private room and dedicated equipment should be used in patient care, with terminal equipment disinfection by the appropriate agency facility. Room assignments should be arranged to avoid cross-contamination whenever possible. Individuals with symptoms of respiratory infection should be prevented from caring for or visiting pediatric, immunocompromised, or cardiac patients.

The administration of high doses of respiratory syncytial virus immune globulin is an effective means of preventing lower respiratory tract infection in infants and young children at high risk for contracting this disease. Palivizumab, a monoclonal antibody given intramuscularly, can prevent RSV disease in high-risk infants and children.

Rift Valley virus

A Phlebovirus that causes sporadic epidemics of hemorrhagic fever among humans and animals in Africa. It is transmitted by the bite of infected mosquitoes.

RNA virus

A virus such as the HIV, influenza virus, and polio virus whose genome is ribonucleic acid (RNA).

Ross River virus

An Alphavirus transmitted by mosquito bite that causes fevers, rash, and epidemic arthritis in multiple joints. It is typically found in Australia and neighboring islands.

Sabiá virus

An arenavirus that causes Brazilian hemorrhagic fever. The reservoir for the virus is unknown. Ribavirin, which is effective against Lassa fever, may be effective against Brazilian hemorrhagic fever.

sandfly fever virus

Toscana virus

Serra do Navio virus

Abbreviation: SDNV
A California serogroup virus of the Bunyaviridae family. It causes a febrile encephalitis.

simian immunodeficiency virus

A family of HIV-like retroviruses that primarily infects African green monkeys, in whom it produces an HIV/AIDS-like suppression of immunity.

Sindbis virus

An Alphavirus typically found in South Africa or Oceania that is disseminated to humans by mosquitoes of the genus Culex. It can cause a transient febrile illness accompanied by a diffuse maculopapular rash and muscle and joint pains.

slow virus

A virus that replicates and causes disease indolently.
See: slow virus infection

street virus

A rabies virus obtained from an infected animal rather than from a laboratory strain.

SV 40 virus

Simian virus 40, a member of the Papovavirus family. The virus produces sarcomas after subcutaneous inoculation into newborn hamsters.

Tacaribe complex virus

A group of viruses, originally identified in South America, that cause hemorrhagic fever in humans. They are members of the Arenavirus family and are typically found in rodents. One member of this group is the Sabiá virus.

Tahyna virus

Abbreviation: TAH
A European arbovirus of the Bunyaviridae family. It is transmitted to humans by mosquito bite and causes fevers, respiratory illnesses, encephalitis, and meningitis.

Toscana virus

A Bunyavirus transmitted by insect bite, esp. the bite of the sandfly (Phlebotomus papatasi). The virus is endemic in Sicily, Cyprus, and elsewhere in the Mediterranean and may cause encephalitis, aseptic meningitis, or septicemia.
Synonym: sandfly fever virus

transfusion-transmissible virus

Abbreviation: TTV
A single-stranded DNA virus, found in recipients of blood transfusions, that colonizes in the liver. It is not known whether the virus causes liver disease, e.g., chronic hepatitis, or benignly colonizes the liver.

tanapox virus

A double-stranded DNA poxvirus that occasionally infects humans, causing a pustular, nodular rash, fever, headache, and other symptoms. It is related to other orthopoxviruses (which cause smallpox, monkeypox, and vaccinia).

tumor virus

A virus that causes malignant neoplasms. Viruses suspected of causing tumors in humans include Epstein-Barr virus (associated with Burkitt lymphoma), hepatitis B virus (associated with hepatocellular carcinoma), papilloma virus (associated with carcinoma of the cervix), and human herpesvirus 8 (associated with Kaposi sarcoma).
Synonym: oncogenic virus

vaccinia virus

A double-stranded DNA virus, the causative agent of cowpox and a member of the Orthopoxvirus genus. Vaccines against smallpox are derived from live cultures of vaccinia virus. See: Orthopoxvirus

varicella-zoster virus

The herpesvirus that causes chickenpox and shingles.
Synonym: human herpesvirus 3
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VECTOR OF WEST NILE VIRUS: The Culex mosquito, vector of West Nile virus

West Nile virus

A Flavivirus that primarily infects birds but can be transmitted by mosquito bite to humans and other animals. Since West Nile virus was identified in the U.S. in 1999, it has produced a nationwide epidemic of encephalitis. Although infection is usually asymptomatic, signs and symptoms that are more likely to be observed in the very young, the very old, or the very sick include fever, headache, stiff neck, fatigue, loss of appetite, nausea or vomiting, muscle pain, aches, and weakness (“neuroinvasive” disease may produce an acute flaccid paralysis). Infection is occasionally fatal. Those over age 50 are at greatest risk for serious complications and death.

In 2009 45 states in the U.S. reported having human cases of West Nile fever. There were 720 reported cases of this viral infection in the U.S. in 2009 and 32 fatalities. Infected patients with neuroinvasive disease sometimes suffer long-term consequences of infection, including fatigue and malaise, difficulty concentrating or thinking, or movement disorders. The disease is sometimes spread from patient to patient by blood transfusion or organ transplantation.

Patient care

Disease transmission can be prevented with mosquito control and mosquito avoidance measures. Health care professionals should advise patients and families to limit time out of doors, esp. at dusk and dawn, to wear protective clothing (long sleeves, long pants, and socks), to place mosquito netting over infant carriers or strollers, and to apply an FDA-approved insect repellant (e.g., DEET, picaridin, or oil of lemon eucalyptus). Mosquito breeding grounds should be eliminated: standing water should be removed from flower pots, bird baths, pool covers, rain gutters, and discarded tires. Window and door screens should be installed and kept in good repair to prevent mosquitoes from entering homes.

See: illustration

xenotropic murine leukemia virus–related virus

Abbreviation: XMRV virus
A human retrovirus that has been isolated from some patients with chronic fatigue syndrome and prostate cancer.
Genetic material
RNAHIV, hepatitis A, polio, measles, mumps, rhinovirus, influenza
DNAHerpesviruses, hepatitis B, adenoviruses, human papilloma viruses, cytomegalovirus
HumansMeasles, mumps, rubella, varicella-zoster, poliovirus
Humans and animalsRabies, influenza, hantavirus, encephalitis virus
PlantsTobacco mosaic virus, cowpea mosaic virus
PresentHerpesviruses, rabies, HIV
AbsentRotavirus, Norwalk virus, adenovirus
RespiratoryInfluenza, parainfluenza, hantavirus
TeratogenicVaricella-zoster virus, cytomegalovirus, rubella
Neurological and fatalRabies
Paralytic encephaliticPolio, many encephalitis viruses
FulminantYellow fever, hantavirus, Ebola-Marburg
Cancer causingHuman T-cell lymphotrophic virus, hepatitis viruses, papillomavirus
Medical Dictionary, © 2009 Farlex and Partners


a submicroscopic intracellular parasite, ranging in size from about 0.025 μ m to 0.25 μ m. The mature virus, a virion, consists of nucleic acid (RNA or DNA, which can be single-stranded or double-stranded, circular, linear or in separate segments, see SEGMENTED GENOME) surrounded by a coat of protein (capsid). In some viruses this is surrounded by an envelope of lipids, proteins and carbohydrates, and there may be spikes. Viruses infect cells of microorganisms, plants and animals, and whilst they carry out no METABOLISM themselves, they are able to control the metabolism of the infected cell. In infection the virus interacts with the host cell surface, and penetrates the cell. Virus nucleic acid is replicated and virus components are synthesized inside the cell. The newly formed virions are released from the cell.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005


A tiny particle that can cause infections by duplicating itself inside a cell using the cell's own software. Antibiotics are ineffective against viruses, though antiviral drugs exist for some viruses, including chickenpox.
Gale Encyclopedia of Medicine. Copyright 2008 The Gale Group, Inc. All rights reserved.


A submicroscopic (20-600 nm in diameter) particle (called a virion), which typically contains a protein coat (called a capsid) surrounding genetic material in the form of a double or a single strand of RNA or DNA. Viruses replicate only within cells of living hosts. They can infect cells and are the cause of various diseases. This is accomplished by releasing the viral genetic material into the host cytoplasm if it is RNA, and into the host nucleus if it is DNA, and thus inducing the production of new viral particles and newly infected cells. There are many viruses: DNA viruses as for example the adenovirus (some of which can cause epidemic conjunctivitis), herpesvirus and pox viruses, and RNA viruses as for example the picorna virus (e.g. hepatitis A), toga viruses (e.g. rubella), corona viruses (which can cause respiratory infection) and the retroviruses (e.g. HIV). See antiviral agents; herpesvirus; gene therapy.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann


, pl. viruses (vī'rŭs, -ĕz)
1. Formerly, the specific agent of an infectious disease.
2. Term for a group of infectious agents, which, with few exceptions, are capable of passing through fine filters that retain most bacteria, are usually not visible through the light microscope, lack independent metabolism, and are incapable of growth or reproduction apart from living cells.
3. Relating to or caused by a virus, as a viral disease.
[L. poison]
Medical Dictionary for the Dental Professions © Farlex 2012

Patient discussion about virus

Q. Is there a connection between Epstein-Barr Virus and Fibromyalgia and where can I find information? I was diagnosed with EBV 10 years ago and got diagnosed with Fibromyalgia and Depression 5 yrs ago-is there a connection and if so where can I obtain information. If anyone can help-Thank You!

A. EBV is a herpes virus. And there are arguments from both sides if it has a connection or not to Fibromyalgia, some articles say that , some say the other. The truth is that for the patient it doesn’t mater. If you have it you have to deal with it and it doesn’t mater how it showed up. This mater to the researches.

Q. can an HIV virus last when it comes in contact with air and out of the body? in what ways can i get HIV? and what are the treatment options ? is it treatable ? and what are the side effects for this kind of treatment ?

A. This virus may be passed from one person to another when infected blood, semen, or vaginal secretions come in contact with an uninfected person’s broken skin or mucous membranes*. In addition, infected pregnant women can pass HIV to their baby during pregnancy or delivery, as well as through breast-feeding. People with HIV have what is called HIV infection. Some of these people will develop AIDS as a result of their HIV infection. Hope this helps.

Q. One of my uncle is recovering from the flu and lower back pain. The flu was short... One of my uncle is recovering from the flu and lower back pain. The flu was short lived - this past saturday and super bowl sunday. He felt much better on Monday and worked half a day. Monday night the pain in his back started and he thought he must have twisted wrong somehow. He took a hot bath and went to bed. The next morning, He couldn't move. He immediately went to the doctor. The doctor was a bit puzzled by the pain, but not surpised by it, because of the flu. He said there is definitely a correlation. He was completely debilitated for one entire day, laying on the floor in each room of his house. He was given an inflamatory pill and a muscle relaxer. One day later, today, He is 25% better, which tells him that its a virus and not because he pulled something. He can walk a bit without pain and know tomorrow will be even better. To all who have this problem - Hang in there! and please help!

A. Hey good luck, don't even bother going to the doctor about the back pain because my family has had the flu going around for 2 months now and I think 6 people have had it so far an they all have had the exact same back pain you are talking about and then their stomach starts hurting and they get alot better with in a week later and they are fine so if you can do as much resting as possible.

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