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Viruses are the smallest parasites, ranging from 0.02 to 0.3 μm. They depend completely on cells (bacterial, plant, or animal) to reproduce. Viruses have an outer cover of protein, and sometimes lipid, and an RNA or DNA core. For infection to occur, the virus first attaches to the host cell. The viral DNA or RNA then separates from the outer cover (uncoating) and replicates inside the host cell in a process that requires specific enzymes. Most RNA viruses replicate their nucleic acid in the cytoplasm, whereas most DNA viruses do so in the nucleus. The host cell typically dies, releasing new viruses that infect other host cells.
Some infections are asymptomatic or latent. In latent infection, viral RNA or DNA remains in host cells but does not cause disease unless some trigger causes symptom recurrence. Latency may facilitate person-to-person spread. Herpesviruses exhibit latency.
Several hundred different viruses infect humans (see Table 1: Viruses: Selected Viruses That Infect Humans*  ). Viruses that primarily infect humans often spread via respiratory and enteric excretions. Some are transmitted sexually and through transfer of blood. Viruses exist worldwide, but their spread is limited by inborn resistance, prior immunizing infections or vaccines, sanitary and other public health control measures, and prophylactic antiviral drugs.
Zoonotic viruses pursue their biologic cycles chiefly in animals; humans are secondary or accidental hosts. These viruses are limited to areas and environments able to support their nonhuman natural cycles of infection (vertebrates, arthropods, or both). Most are discussed in Arboviridae, Arenaviridae, Bunyaviridae, and Filoviridae.
Some viruses are oncogenic. Human T-lymphotropic virus 1 predisposes to human leukemia and lymphoma. Epstein-Barr virus predisposes to malignancies such as nasopharyngeal carcinoma, Burkitt's lymphoma, Hodgkin lymphoma, and lymphomas in immunosuppressed organ transplant recipients. Hepatitis B and C viruses predispose to hepatocellular carcinoma. Human herpesvirus 8 predisposes to Kaposi's sarcoma, primary effusion lymphomas, and Castleman disease (a lymphoproliferative disorder).
Slow viral diseases have lengthy incubations (months or years) and are often due to reactivation of a virus that caused an earlier infection. They cause some chronic degenerative diseases, including subacute sclerosing panencephalitis (measles virus), progressive rubella panencephalitis, and progressive multifocal leukoencephalopathy (JC virus). Creutzfeldt-Jakob disease and bovine spongiform encephalopathy have characteristics similar to slow viral diseases, but they are caused by prions (see Brain Infections: Prion Diseases).
Diagnosis
Some viral diseases can be diagnosed clinically or epidemiologically (eg, by well-known viral syndromes such as measles, rubella, roseola infantum, erythema infectiosum, and chickenpox, or during epidemic outbreaks such as influenza). For others, definitive diagnosis is necessary mainly when specific treatment may be helpful or when the agent may be a public health threat (eg, severe acute respiratory syndrome [SARS]). Such cases require testing.
Serologic examination during acute and convalescent stages is sensitive and specific but slow; more rapid diagnosis can sometimes be made using culture, PCR, or viral antigen tests. Histopathology with electron (not light) microscopy can sometimes help. Specific diagnostic procedures are discussed in Laboratory Diagnosis of Infectious Disease. For many less common diseases (eg, rabies, smallpox, Eastern equine encephalitis), state health laboratories and the Centers for Disease Control and Prevention can analyze specimens.
Treatment
and Prevention
Antiviral drugs:
Progress in the use of antiviral drugs is occurring rapidly. Antiviral chemotherapy can be directed at various phases of viral replication. It can interfere with viral particle attachment to host cell membranes or uncoating of viral nucleic acids, inhibit a cellular receptor or factor required for viral replication, or block specific virus-coded enzymes and proteins produced in the host cells that are essential for viral replication but not for normal host cell metabolism. Antiviral drugs are most often used therapeutically or prophylactically against herpesviruses (including cytomegalovirus—see Herpesviruses), respiratory viruses (see Respiratory Viruses), and HIV (see Human Immunodeficiency Virus (HIV)). However, some drugs are effective against many different kinds of viruses. Some drugs against HIV are being evaluated for other viral infections such as hepatitis B virus (HBV).
Interferons:
Interferons are compounds released from infected host cells in response to viral or other foreign antigens. There are many different interferons, which have numerous effects that include blocking translation and transcription of viral RNA and stopping viral replication without disturbing normal host cell function. Interferons are sometimes given attached to polyethylene glycol (pegylated formulations), which allow a slow, sustained release of the interferon.
Interferon therapy for viral infection is used for hepatitis B and C and human papillomavirus. Interferon is indicated for patients with chronic infection with HBV or hepatitis C virus (HCV) plus abnormal liver function tests and either detectable viral loads or biopsy-documented active disease. Interferon-α2b can be used as a treatment for HBV at doses of 5 million units sc once/day or 10 million units sc 3 times/wk for 16 wk; treatment may induce clearance of HBV DNA and the hepatitis B e antigen (HBeAg) from serum and improve liver function tests and liver histology. Higher doses are required if there is coexistent delta hepatitis. HCV is treated with ribavirin plus either pegylated interferon-α2b 1.5 μg/kg sc once/wk or pegylated interferon-α2a 180 μg sc once/wk; treatment may decrease HCV RNA level and improve liver function and liver histology. Interferon-α-n3, intralesionally or IM, has also cleared intractable condyloma acuminata of skin and genitals, but its optimal administration and long-term effects are unclear. A recombinant form of endogenous interferon-α is being studied in hairy cell leukemia, Kaposi's sarcoma, human papillomavirus, and respiratory viruses.
Adverse effects include fever, chills, weakness, and myalgia, typically starting 7 to 12 h after the 1st injection and lasting up to 12 h. Depression, hepatitis, and, when used at high doses, bone marrow suppression are also possible.
Vaccines
and immune globulins:
Vaccines work by stimulating native immunity. Viral vaccines in general use include influenza, measles, mumps, poliomyelitis, rabies, rubella, hepatitis A, hepatitis B, varicella, and yellow fever. Adenovirus and smallpox vaccines are available but used only in high-risk groups (eg, military recruits).
Immune globulins are available for passive immune prophylaxis in limited situations. Some are used postexposure (eg, rabies, hepatitis immune globulins). Others may help in treating disease.
Last full review/revision November 2005
Content last modified November 2005
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