virus
[vi´rus] 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.
bacterial virus one that is capable of producing transmissible lysis of bacteria; see also
bacteriophage.
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 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-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.
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.
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.
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.
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 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.
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.
virus
/vi·rus/ (vi´rus) [L.] a minute infectious agent which, with certain exceptions, is not resolved by the light microscope, lacks independent metabolism and is able to replicate only within a living host cell; the individual particle (virion) consists of nucleic acid (nucleoid)—DNA or RNA (but not both)—and a protein shell (capsid), which contains and protects the nucleic acid and which may be multilayered.
attenuated virus one whose pathogenicity has been reduced by serial passage or other means.
BK virus (BKV) a human polyomavirus that causes widespread infection in childhood and remains latent in the host; it is believed to cause hemorrhagic cystitis and nephritis in immunocompromised patients.
Central European encephalitis virus a species of tick-borne viruses of the genus
Flavivirus that includes the agents of Central European encephalitis and the Russian spring-summer encephalitis virus.
cowpox virus a virus of the genus
Orthopoxvirus that is the etiologic agent of
cowpox.
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
helper v. dengue virus a flavivirus existing as four distinct types (designated 1, 2, 3, and 4) that causes dengue.
DNA virus one whose genome consists of DNA.
Ebola virus 1. an RNA virus almost identical to the Marburg virus but serologically distinct; it causes a similar disease.
encephalomyocarditis virus an enterovirus that causes mild aseptic meningitis and encephalomyocarditis.
enteric viruses an epidemiologic class of viruses that are normally acquired by ingestion and replicate in the intestinal tract, causing local rather than generalized infection.
enveloped virus a virus having an outer lipoprotein bilayer acquired by budding through the host cell membrane.
Epstein-Barr virus (EBV)
human herpesvirus 4; a virus that causes infectious mononucleosis and is associated with Burkitt's lymphoma and nasopharyngeal carcinoma.
equine encephalomyelitis virus a group of arbovirus species of the genus
Alphavirus that cause encephalomyelitis in horses, mules, and humans, transmitted by mosquitoes; there are three strains:
eastern, western, and
Venezuelan. fixed virus one whose virulence and incubation period have been stabilized by serial passage and remained fixed during further transmission.
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 virus the etiologic agent of viral hepatitis. Six types are recognized:
hepatitis A virus, the agent causing infectious hepatitis, acquired by parenteral inoculation or by ingestion;
hepatitis B virus, the agent causing serum hepatitis, transmitted by inadequately sterilized syringes and needles, or through infectious blood plasma, or certain blood products;
hepatitis C virus, which causes hepatitis C;
hepatitis D virus, a defective RNA viral agent that can replicate only in the presence of hepatitis B virus and is transmitted with it and causes hepatitis D;
hepatitis E virus, a calicivirus transmitting hepatitis E; and
hepatitis G virus, flavivirus isolated from patients with hepatitis but whose etiologic role is uncertain.
herpes simplex virus (HSV) a virus of the genus
Simplexvirus that is the etiologic agent of herpes simplex in humans. It is separable into two serotypes, designated 1 and 2 (called also
human herpesvirus 1 and
human herpesvirus 2 ); type 1 is transmitted by infected saliva and causes primarily nongenital lesions, and type 2 is sexually transmitted and causes primarily genital lesions.
human T-lymphotropic virus 2 (HTLV-2) a species of retroviruses having extensive serologic cross-reactivity with HTLV-1; no clear association with disease has been established.
igbo-ora virus an arbovirus of the genus
Alphavirus that has been associated with a dengue-like disease in Nigeria, the Central African Republic, and the Ivory Coast.
influenza virus any of a group of orthomyxoviruses that cause influenza, including at least three genera:
Influenzavirus A,
Influenzavirus B, and
Influenzavirus C. Serotype A viruses are subject to major antigenic changes (antigenic shifts) as well as minor gradual antigenic changes (antigenic drift) and cause the major pandemics.
Jamestown Canyon virus a virus of the genus
Bunyavirus, serologically related to California encephalitis virus, that occasionally causes encephalitis.
JC virus (JCV) a polyomavirus that causes widespread infection in childhood and remains latent in the host; it is the cause of progressive multifocal leukoencepahalopathy.
La Crosse virus a virus of the California serogroup of the genus
Bunyavirus, the etiologic agent of La Crosse encephalitis.
lymphocyte-associated virus any virus of the subfamily Gammaherpesviridae, members of which are specific for either B or T lymphocytes; infection is often arrested at a lytic or prelytic stage without production of infectious virions, and latent virus may frequently be demonstrated in lymphoid tissue.
lytic virus one that is replicated in the host cell and causes death and lysis of the cell.
Marburg virus an RNA virus occurring in Africa, transmitted by insect bite and causing Marburg disease.
masked virus a virus that ordinarily occurs in a noninfective state and is demonstrable by indirect methods which activate it, as by blind passage in experimental animals.
monkeypox virus an orthopoxvirus that produces mild exanthematous disease in monkeys and a smallpox-like disease in humans.
mumps virus a virus of the genus
Rubulavirus that causes
mumps and sometimes tenderness and swelling of the testes, pancreas, ovaries, or other organs.
naked virus , nonenveloped virus a virus lacking an outer lipoprotein bilayer.
neurotropic virus one that has a predilection for and causes infection in nervous tissues, e.g., the rabies virus.
Norwalk virus a calicivirus that is a common agent of epidemics of acute gastroenteritis.
oncogenic viruses 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.
Oropouche virus a virus of the genus
Bunyavirus that causes illness in Brazil; infection is characterized by fever, chills, malaise, headache, myalgia, and arthralgia, sometimes nausea and vomiting, and occasionally central nervous system involvement.
orphan viruses viruses isolated in tissue culture but not found specifically associated with any illness.
parainfluenza virus a group of viruses of the family Paramyxoviridae that cause upper respiratory tract disease in humans and other animals.
Powassan virus a tickborne virus of the genus
Flavivirus that causes encephalitis in the eastern United States and Canada.
pseudocowpox virus a virus of the genus Parapoxvirus that produces nodular lesions similar to those of cowpox and orf on the udders and teats of milk cows and the oral mucosa of suckling calves (paravaccinia), which can be transmitted to humans during milking.
rabies virus an RNA virus of the rhabdovirus group that causes rabies.
respiratory viruses an epidemiologic class of viruses that are acquired by inhalation of fomites and replicate in the respiratory tract, causing local rather than generalized infection; they are included in the families Adenoviridae, Coronaviridae, Orthomyxoviridae, Paramyxoviridae, and Picornaviridae.
respiratory syncytial viruses (RSV) viruses belonging to the genus
Pneumovirus, causing respiratory disease that is particularly severe in infants, and in tissue causing
syncytium formation.
RNA virus one whose genome consists of RNA.
Rous-associated virus (RAV) a helper virus in whose presence a defective Rous sarcoma virus is able to form a protein coat.
rubella virus the sole species of the genus
Rubivirus, the etiologic agent of rubella.
St. Louis encephalitis virus a virus of the genus
Flavivirus, that is the etiologic agent of St. Louis encephalitis; transmitted by mosquitoes.
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 immunodeficiency virus (SIV) a virus of the genus
Lentivirus, closely related to human immunodeficiency virus, that causes inapparent infection in African green monkeys and a disease resembling acquired immunodeficiency syndrome in macaques.
slow virus any virus causing a disease characterized by a long preclinical course and gradual progression once the symptoms appear.
street virus virus from a naturally infected animal, as opposed to a laboratory-adapted strain of the virus.
tanapox virus a virus of the genus
Yatapoxvirus that is the etiologic agent of tanapox.
Toscana virus a virus of the Naples serogroup of the genus
Phlebovirus, an etiologic agent of phlebotomus fever.
variola virus the virtually extinct virus, belonging to the genus 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 that causes West Nile encephalitis; it is transmitted by
Culex mosquitoes, with wild birds serving as the reservoir.
Yaba monkey tumor virus a virus of the genus
Yatapoxvirus that is the etiologic agent of yabapox.
yellow fever virus a mosquito-borne species of the genus
Flavivirus that causes yellow fever in Central and South America and Africa.
virus
(vi'rus) [L. virus, poison] 

VIRAL SHAPES


VIRAL SHAPES


VIRAL SHAPES
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
Classification
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).
Treatment
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
Bacteriophage.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: coxsackieviruscytomegalic virus
Abbreviation: CMV
Cytomegalovirus.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 DDNA virus
A virus such as the papilloma virus and the herpesviruses whose genome is deoxyribonucleic acid (DNA).
EB virus
Epstein-Barr virus.enteric virus
Enterovirus.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 virusEpstein-Barr virus
See: Epstein-Barr virusEyach 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 filterfixed virus
A rabies virus stabilized and modified but only partially attenuated by serial passage through rabbits.
foamy virus
Spumavirus.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 virushepatitis 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.
Treatment
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.
CAUTION!
Caregivers with active oral or cutaneous lesions should avoid providing patient care.
herpes virus
See: herpesvirus

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.


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 syndromehuman papilloma virus
See: papillomavirushuman 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: influenzaInkoo 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:
CalicivirusOliveros 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
Poxvirus.Rauscher leukemia virus
See: Rauscher leukemia virusreassortant 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.
Symptoms
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
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 virusSerra 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 infectionstreet 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 virustransfusion-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 virusvaccinia 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:
Orthopoxvirusvaricella-zoster virus
The herpesvirus that causes chickenpox and shingles.
Synonym: human herpesvirus 3

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: illustrationxenotropic 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.
Characteristics | Examples |
---|
Genetic material | |
RNA | HIV, hepatitis A, polio, measles, mumps, rhinovirus, influenza |
DNA | Herpesviruses, hepatitis B, adenoviruses, human papilloma viruses, cytomegalovirus |
Hosts | |
Humans | Measles, mumps, rubella, varicella-zoster, poliovirus |
Humans and animals | Rabies, influenza, hantavirus, encephalitis virus |
Plants | Tobacco mosaic virus, cowpea mosaic virus |
Bacteria | Phages |
Envelope | |
Present | Herpesviruses, rabies, HIV |
Absent | Rotavirus, Norwalk virus, adenovirus |
Respiratory | Influenza, parainfluenza, hantavirus |
Teratogenic | Varicella-zoster virus, cytomegalovirus, rubella |
Neurological and fatal | Rabies |
Paralytic encephalitic | Polio, many encephalitis viruses |
Fulminant | Yellow fever, hantavirus, Ebola-Marburg |
Latent | Herpesviruses |
Cancer causing | Human T-cell lymphotrophic virus, hepatitis viruses, papillomavirus |