Introduction
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, enters it, and releases its DNA or RNA inside the cell. The virus's DNA or RNA is the genetic code containing the information needed to replicate the virus. The viral 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. Before it dies, however, the cell releases new viruses, which go on to infect other cells.
Some viruses do not kill the cells they infect, but instead alter the cells' functions. Sometimes the infected cell loses control over normal cell division and becomes cancerous. Some viruses that do not kill the cells they infect leave their genetic material in the host cell where it remains dormant for an extended time (latent infection). When the cell is disturbed, the virus may be able to begin growing 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.
Viruses are transmitted in a variety of ways. Some are swallowed, some are inhaled, and some are transmitted by the bites of insects and other parasites (for example, mosquitoes and ticks).
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 noninfected cells more resistant to infection by many viruses.
Upon entering the body, a virus triggers the body's immune defenses. These defenses begin with white blood cells, such as lymphocytes, which learn to attack and destroy the virus or the cells it has infected (see Section 16, Chapter 183). If the body survives the virus attack, the lymphocytes "remember" the invader and are able to respond more quickly and effectively to a subsequent infection by the same virus. This is called immunity. Immunity can also be produced by receiving a vaccine.
Drugs that combat viral infections are called antiviral drugs. Antiviral drugs work by interfering with viral replication. Because viruses are tiny and replicate inside cells using the cells' own metabolic pathways, 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 against which antibiotics can be directed. Therefore, antiviral drugs are much more difficult to develop. Antiviral drugs can be toxic to human cells. Viruses can develop resistance to antiviral drugs.
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.
Probably the most common viral infections are those of the nose, throat, and airways. These infections include sore throat, sinusitis, the common cold, and influenza. Doctors often refer to these as upper respiratory infections (URIs). In small children, viruses also commonly cause croup and inflammation of the windpipe (laryngitis) or other airways deeper inside the lungs (bronchiolitis, bronchitis (see Section 23, Chapter 274)).
Some viruses (for example, rabies, West Nile virus, and several different encephalitis viruses) infect the nervous system (see Section 6, Chapter 89). Viral infections also develop in the skin, sometimes resulting in warts or other blemishes (see Section 18, Chapter 213). Additionally, many viruses commonly infect infants and children (see Section 23, Chapter 273).
Other common viral infections are caused by the herpesviruses. Eight different herpesviruses infect people. Three of these--herpes simplex virus type 1, herpes simplex virus type 2, and varicella-zoster virus--cause infections that produce blisters on the skin. 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 an illness similar to infectious mononucleosis in people with a healthy immune system. Human herpesviruses 6 and 7 cause a childhood illness known as roseola infantum (see Section 23, Chapter 273). 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.
 See the drug table Antiviral Drugs.
 See the table Viruses and Cancer: A Link.
 See the sidebar Smallpox: New Risks From an Old Disease.
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