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Pronunciations acyclovir amantadine bronchi Burkitt's lymphoma cytomegalovirus encephalitis foscarnet ganciclovir gastrointestinal hantavirus hepatitis immune globulin immunoglobulin Kaposi's sarcoma keratitis lymphocytes lymphoma monocytes mucus papilloma papillomavirus polymerase chain reaction respiratory syncytial virus ribavirin rimantadine roseola infantum trifluridine varicella-zoster vidarabine

A virus is a small infectious organism—much smaller than a fungus or bacterium—that must invade a living cell to reproduce (replicate). The virus attaches to a cell (called the host cell), enters it, and releases its DNA or RNA inside the cell. The virus's DNA or RNA is the genetic material containing the information needed to replicate the virus. The virus's genetic material takes control of the cell and forces it to replicate the virus. The infected cell usually dies because the virus keeps it from performing its normal functions. When it dies, the cell releases new viruses, which go on to infect other cells.

Virus Attaching to a Cell

Some viruses do not kill the cells they infect but instead alter the cell's functions. Sometimes the infected cell loses control over normal cell division and becomes cancerous. Some viruses leave their genetic material in the host cell, where the material remains dormant for an extended time (latent infection). When the cell is disturbed, the virus may begin replicating again and cause disease.

Viruses usually infect one particular type of cell. For example, cold viruses infect only cells of the upper respiratory tract. Additionally, most viruses infect only a few species of plants or animals. Some infect only people. Many viruses commonly infect infants and children (see Viral Infections).

Viruses are spread (transmitted) in various ways. Some are swallowed, some are inhaled, and some are spread by the bites of insects and other parasites (for example, mosquitoes and ticks). Some are spread sexually.

Defenses: The body has a number of defenses against viruses. Physical barriers, such as the skin, discourage easy entry. Infected cells also make interferons, substances that can make uninfected cells more resistant to infection by many viruses.

When a virus enters the body, it triggers the body's immune defenses. These defenses begin with white blood cells, such as lymphocytes and monocytes, which learn to attack and destroy the virus or the cells it has infected (see Biology of the Immune System: Introduction). If the body survives the virus attack, some of the white blood cells remember the invader and are able to respond more quickly and effectively to a subsequent infection by the same virus. This response is called immunity. Immunity can also be produced by getting a vaccine.

Types of Viral Infections: Probably the most common viral infections are those of the nose, throat, and upper airways (upper respiratory infections). These infections include sore throat, sinusitis, and the common cold. Influenza is a viral respiratory infection. In small children, viruses also commonly cause croup and inflammation of the windpipe (laryngotracheobronchitis) or other airways deeper inside the lungs (bronchiolitis—see Respiratory Disorders: Bronchiolitis). Respiratory infections are more likely to cause severe symptoms in infants, older people, and people with a lung or heart disorder.

Some viruses (such as rabies, West Nile virus, and several different encephalitis viruses) infect the nervous system (see Infections of the Brain and Spinal Cord: Introduction). Viral infections also develop in the skin, sometimes resulting in warts or other blemishes (see Viral Skin Infections: Warts).

Other common viral infections are caused by herpesviruses. Eight different herpesviruses infect people. Three of them—herpes simplex virus type 1, herpes simplex virus type 2, and varicella-zoster virus—cause infections that produce blisters on the skin or mucus membranes. Another herpesvirus, Epstein-Barr virus, causes infectious mononucleosis. Cytomegalovirus is a cause of serious infections in newborns and in people with a weakened immune system. It can also produce symptoms similar to infectious mononucleosis in people with a healthy immune system. Human herpesviruses 6 and 7 cause a childhood infection called roseola infantum (see Viral Infections: Roseola Infantum). Human herpesvirus 8 has been implicated as a cause of cancer (Kaposi's sarcoma) in people with AIDS.

All of the herpesviruses cause lifelong infection because the virus remains within its host cell in a dormant (latent) state. Sometimes the virus reactivates and produces further episodes of disease. Reactivation may occur rapidly or many years after the initial infection.

Diagnosis

Common viral infections may be diagnosed based on symptoms. For infections that occur in epidemics (such as influenza), the presence of other similar cases may help doctors identify a particular infection. For other infections, blood tests and cultures (growing microorganisms in the laboratory from samples of blood, body fluid, or other material taken from an infected area) may be done. Blood may be tested for antibodies to viruses or for antigens (proteins on or in viruses that trigger the body's defenses). Polymerase chain reaction (PCR) techniques may be used to make many copies of the viral genetic material, enabling doctors to rapidly and accurately identify the virus. Tests are sometimes done quickly—for instance, when the infection is a serious threat to public health or when symptoms are severe. A sample of blood or other tissues is sometimes examined with an electron microscope, which provides high magnification with clear resolution.

Treatment

Drugs that combat viral infections are called antiviral drugs. Many antiviral drugs work by interfering with replication of viruses. Most drugs used to treat human immunodeficiency virus (HIV) infection (see Human Immunodeficiency Virus (HIV) Infection: Treatment) work this way. Because viruses are tiny and replicate inside cells using the cells' own metabolic functions, there are only a limited number of metabolic functions that antiviral drugs can target. In contrast, bacteria are relatively large organisms, commonly reproduce by themselves outside of cells, and have many metabolic functions that antibacterial drugs (antibiotics) can target. Therefore, antiviral drugs are much more difficult to develop than antibacterial drugs. Antiviral drugs can be toxic to human cells. Viruses can develop resistance to antiviral drugs.

Other antiviral drugs strengthen the immune response to the viral infection. These drugs include several types of interferons, immunoglobulins, and vaccines. Interferon drugs are replicas of naturally occurring substances that slow or stop viral replication. Immune globulin is a sterilized solution of antibodies (also called immunoglobulins) collected from a group of people. Vaccines are materials that help prevent infection by stimulating the body's natural defense mechanisms. Many immune globulins and vaccines are given before exposure to a virus to prevent infection. Some immune globulins and some vaccines, such as those for rabies and hepatitis B, are also used after exposure to the virus to help prevent infection from developing or reduce the severity of infection. Immune globulins may also help treat some established infections and also prevent infection after future exposures to the virus.

Most antiviral drugs can be given by mouth. Some can also be given by injection into a vein (intravenously) or muscle (intramuscularly). Some are applied as ointments, creams, or eye drops or are inhaled as a powder.

Antibiotics are not effective against viral infections, but if a person has a bacterial infection in addition to a viral infection, an antibiotic is often necessary.

Antiviral Drugs Drug Common Uses Side Effects Acyclovir Genital herpes Herpes zoster (shingles) Chickenpox Few serious side effects: Nausea Vomiting Diarrhea Headache Rashes Kidney damage (rare) Confusion (rare) Amantadine Influenza A Nausea or loss of appetite Nervousness Light-headedness Unsteadiness Sleeplessness Confusion Cidofovir Cytomegalovirus infections Kidney damage Low white blood cell count Famciclovir Genital herpes Herpes zoster (shingles) Chickenpox Few serious side effects (similar to acyclovir) Foscarnet Cytomegalovirus infections Herpes simplex virus infections Kidney damage Electrolyte disturbances Seizures Ganciclovirt Cytomegalovirus infections Low white blood cell count Anemia Interferon-alpha Hepatitis B and C Flu-like symptoms Depression Low white blood cell count Anemia Low platelet count Oseltamivir Influenza A and B Nausea and vomiting Dizziness Penciclovir (cream) Cold sores Few side effects Headache Mild burning or stinging at the site of application Ribavirin Respiratory syncytial virus infection in children Hepatitis C Breakdown of red blood cells, causing anemia Rimantadine Influenza A Similar to amantadine, but usually milder and less common Trifluridine (eye drops) Herpes simplex infection of the cornea (keratitis) Stinging of the eyes Swelling of the eyelids Valacyclovir Genital herpes Herpes zoster (shingles) Chickenpox Few serious side effects (similar to acyclovir) Valganciclovir Cytomegalovirus infections Low white blood cell count Anemia Low platelet count Gastrointestinal symptoms (such as nausea, vomiting, diarrhea, or abdominal pain) Vidarabine (ointment) Herpes simplex keratitis Few side effects Irritation of the eyes Sensitivity to light Zanamivir (inhaled powder) Influenza A and B Irritation of the airways

Viruses and Cancer: A Link Some viruses alter the DNA of their host cells in a way that helps cancer develop. Only a few viruses are known to cause cancer, but there may be others. Virus Cancer Epstein-Barr virus Burkitt's lymphoma   Certain nose and throat cancers   B-cell lymphomas in people with a weakened immune system (such as those with AIDS) Hepatitis B and C viruses Liver cancer Herpesvirus 8 Kaposi's sarcoma in people with AIDS   Non-Hodgkin lymphoma in people with AIDS Human papillomavirus Cervical cancer

Last full review/revision February 2007 by Marguerite Urban, MD