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.
one that is capable of producing transmissible lysis of bacteria; see also bacteriophage
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
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
a virus whose genome
consists of DNA.
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.
(EBV) a herpeslike virus that causes infectious mononucleosis and is associated with Burkitt's lymphoma and nasopharyngeal carcinoma; see also epstein-barr 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.
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.
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.
that is the etiologic agent of a type of pneumonia
an RNA virus occurring in Africa, transmitted by insect bites, and causing marburg virus disease
that causes mumps
and sometimes tenderness and swelling of the testes, pancreas, ovaries, or other organs.
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.
any of various paramyxoviruses
that cause upper respiratory tract disease of varying severity.
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.
a virus whose genome
consists of RNA.
a strain of virus unable to replicate except in the presence of helper virus
; considered to be deficient in coding for capsid
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.
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.
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.