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Abbreviation for hepatitis delta virus.
Farlex Partner Medical Dictionary © Farlex 2012


1. hepatitis D virus
2. high definition video
The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.


(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
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