viral infection, any of the diseases caused by one of approximately 200 viruses pathogenic to humans. Some are the most communicable and dangerous diseases known; some cause mild and transient conditions that pass virtually unnoticed. If cells are damaged by the viral attack, disease exists. The signs of the infection reflect the anatomic location of the damaged cells. Viruses are introduced into the body through nonintact skin or mucous membranes or through a transfusion into the bloodstream or transplantation, by droplet infection through the respiratory tract, or by ingestion through the digestive tract into the GI system. The pathogenicity of the particular virus depends on the rapidity of replication, the enzymes released, the part of the body infected, and the particular action of the virus. After it enters the body, the virus attaches to and enters a cell. The virus directs the cell to produce new virions, using chemical building blocks and energy available in the parasitized cell. The virus has now taken over the cell. After a variable period of time, masses of fully grown viruses appear, each able to survive outside the cell until more susceptible cells are found. In poliovirus infection, one parasitized cell may produce more than 100,000 poliovirus particles in a few hours. Techniques used in viral identification and immunization are based on the essential fact that viruses can multiply only inside living cells. Inoculation of susceptible animals, tissue culture media, and chick embryos allows cultivation of viruses for study and identification and for the preparation of vaccine. Other techniques can also be used in the diagnosis of the cause of viral infection, including serological tests, fluorescent antibody microscopic examination, microscopic examination, and skin tests. In many viral diseases, including mumps, smallpox, and measles, one attack confers permanent immunity. In others, immunity is short-lived. The incubation period for viral infection is usually short, the viruses do not circulate in the bloodstream, antibodies do not form, and most often immunity does not develop. Exposure to a few viruses results in immunity to that virus and to other closely related viruses. Some vectors are able to spread several viruses, but only one at a time. Mechanisms of natural resistance to viral infection are poorly understood, but susceptibility to a particular virus is somehow species-specific; for example, chickenpox, caused by the varicella zoster virus, is seen only in humans. A protective substance, interferon, is elaborated naturally in small amounts in the body. It is cell-specific and species-specific but not virus-specific. Interferon may act as a broad-spectrum antiviral agent, protecting the body from the effects of many viral infections and stopping the synthesis of viral nucleic acid within the parasitized cell. Also called viral disease. See also specific viral infections.
