Viruses, the smallest of all parasites, can live, grow, and reproduce only within the living cells of a host organism, which may be any plant or animal, including human beings. The world is populated by many billions of viruses; most are harmless, but others are the sources of our most persistent and even deadly diseases everything from the common cold to AIDS. In recent years, viral illnesses have gained new attention from researchers because viruses undergo genetic changes, or mutations, and new viral diseases keep cropping up. Researchers are especially concerned by the rapidly increasing global range and speed with which disease bearing viruses can spread, due, at least in part, to wars, population density, human migration, deforestation, and air travel. Epidemiologists are attempting to establish an international warning system to spot newly evolving viral diseases quickly, so that they can be contained and treated when possible. These efforts were spurred by the 1995 outbreak in Zaire of the deadly Ebola virus, which causes widespread internal hemorrhaging. There is mounting evidence that certain viruses may trigger a number of chronic diseases, including multiple sclerosis, Alzheimer’s disease, and juvenile onset, or Type I, diabetes. Viruses are also implicated in some malignancies. Cervical cancer, for example, is associated with the papilloma virus, which causes genital warts. Hepatitis C increases the risk of liver cancer; HIV appears to cause AIDS related lymphoma in some AIDS patients and the herpes virus is implicated in Kaposi’s sarcoma, another AIDS related cancer. New insight into the possible viral origins of such diseases may ultimately lead to new vaccines and treatments. In humans, viral infections are spread mainly by blood and respiratory and intestinal secretions. They may also be sexually transmitted or carried from one person to another by a biting insect. (Two examples: epidemics of dengue, or breakbone fever, have been traced to a mosquito originating in Hawaii; the Asian tiger mosquito, which entered the United States in 1986, is responsible for spreading the Eastern equine encephalitis virus.) Although viral diseases differ greatly in severity and duration, they follow similar patterns of infection and replication. Once a virus invades the body, it enters its host’s target cells. Some attach themselves to receptors on the cell’s surface; others have proteins that allow them to penetrate the cell. Once inside, the virus sheds its coat or undergoes other changes that allow it to insert its genetic material into that of the host cell and then reproduce many times. Eventually, the new viruses escape from the host cell and travel through the body seeking new target cells. Depending upon the virus, they may travel through the bloodstream, the lymph system, or along nerve path ways until reaching their target organ. Much remains to be learned about viral infections and their consequences. With so called slow viruses, such as HIY, for example, there are no symptoms until months or years after infection. Other viruses cause acute, but self limiting illnesses; colds, flu, and measles are common examples. Still others, such as the varicella zoster virus, which causes chickenpox and shingles, seem to disappear, but may reemerge months or years later in somewhat different manifestations. Fortunately, the body has many defenses against viruses, beginning with the skin. Those that enter through the respiratory or gastrointestinal tract face attack from the body’s immune defenses as well as chemical and structural barriers. As soon as a virus enters the body, the immune system begins to make antibodies against it. This response does not always halt infection, but it may result in immunity against future attacks. A bout of rubella or measles, for example, confers lifelong immunity against these diseases.

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