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 enters the host cell and 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 replicate in the nucleus. The host cell typically dies, releasing new viruses that infect other host cells.
The consequences of viral infection vary considerably. Many infections cause acute illness after a brief incubation period, but some are asymptomatic or cause minor symptoms that may not be recognized except in retrospect. Many viral infections are cleared by the body's defenses, but some remain in a latent state. In latent infection, viral RNA or DNA remains in host cells but does not cause disease for a long time, sometimes for many years. Latent viral infections may be transmissible during the asymptomatic period, facilitating person-to-person spread. Sometimes a trigger (particularly immunosuppression) causes reactivation. Common viruses that remain latent include
Some disorders are caused by viral reactivation in the CNS after a very long latency period. These diseases include progressive multifocal leukoencephalopathy (due to the JC virus, a polyomavirus), subacute sclerosing panencephalitis (due to the measles virus), and progressive rubella panencephalitis (due to rubella virus). Creutzfeldt-Jakob disease and bovine spongiform encephalopathy were formerly termed slow viral diseases because they have lengthy incubations (years), but they are now known to be caused by prions; prions are proteinaceous disease-causing agents that are not bacterial, fungal, or viral and that contain no genetic material (see Prion Diseases: Overview of Prion Diseases).
Several hundred different viruses infect humans. Viruses that infect primarily humans often spread via respiratory and enteric excretions. Some are transmitted sexually and through transfer of blood. Some viruses are transmitted via arthropod vectors. 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 (see Arboviridae, Arenaviridae, and Filoviridae) 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).
Viruses and cancer:
Some viruses are oncogenic and predispose to certain cancers:
Some viral disorders can be diagnosed clinically (eg, by well-known viral syndromes such as measles, rubella, roseola infantum, erythema infectiosum, and chickenpox) or epidemiologically (eg, during epidemic outbreaks such as influenza). Definitive laboratory diagnosis is necessary mainly when specific treatment may be helpful or when the agent may be a public health threat (eg, HIV). Typical hospital laboratories can test for some viruses, but for less common disorders (eg, rabies, Eastern equine encephalitis), specimens must be sent to state health laboratories or the Centers for Disease Control and Prevention.
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. For specific diagnostic procedures, see Laboratory Diagnosis of Infectious Disease.
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 that are produced in the host cells and 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 active against HIV are used for other viral infections such as hepatitis B.
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 such as 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), allowing slow, sustained release of the interferon.
Viral disorders sometimes treated with interferon therapy include
Adverse effects of interferons include fever, chills, weakness, and myalgia, typically starting 7 to 12 h after the first injection and lasting up to 12 h. Depression, hepatitis, and, when high doses are used, bone marrow suppression are also possible.
Vaccines (see Immunization: Overview of Immunization) work by stimulating native immunity. Viral vaccines in general use include hepatitis A, hepatitis B, human papillomavirus, influenza, measles, mumps, poliomyelitis, rabies, rotavirus, rubella, varicella, and yellow fever. Adenovirus and smallpox vaccines are available but used only in high-risk groups (eg, military recruits).
Immune globulins (see Immunization: Passive Immunization) are available for passive immune prophylaxis in limited situations. They can be used preexposure (eg, for hepatitis A), postexposure (eg, for rabies or hepatitis), and for treating disease (eg, eczema vaccinatum).
Many viral infections can be prevented by commonsense protective measures (which vary depending on the transmission mode of a given agent). Important measures include hand washing, appropriate food preparation and water treatment, avoidance of contact with sick people, and safe-sex practices. For infections with an insect vector (eg, mosquitoes, ticks), avoiding the vector is important.
Last full review/revision December 2009 by Kenneth M. Kaye, MD
Content last modified February 2012