Overview

Definition

Viruses are tiny particles smaller than bacteria. After invading an animal or plant, viruses use the organism's cells to reproduce themselves. The plant or animal cell may die as the viral DNA or RNA takes over control of the cell's functions. New copies of the virus are released, which go on to infect other cells to begin the process again.

Every type of human cell is susceptible to a virus of one kind or another. Most viruses are more likely to invade certain types of cells. The nature of the infection depends on the type of virus, the system affected and the host response. Some viruses cause mild symptoms that disappear in a few days, such as the common cold viruses. Others such as influenza (flu) or coronavirus, which causes SARS, may bring on severe or even life-threatening respiratory symptoms.

Viruses cause most respiratory infections, including the common cold. Adults on average suffer from about three or four colds a year. Viruses are also responsible for infections of the nervous system, skin, liver and digestive system. Viruses that attack white blood cells include Epstein Barr virus (infectious mononucleosis), which infects the B lymphocyte and human immunodeficiency virus (HIV), the virus that causes AIDS, which targets the helper T cells that coordinate the immune system.

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This article was reviewed and updated June 2007.