1. Infectious and parasitic microorganism much smaller than a bacterium, too small to be seen with a regular microscope or to be trapped even by ceramic filters. A virus is incapable of independent metabolic activity and replication, it must invade a live animal, human, or plant cell from which to derive the energy for survival. For replication, the virus provides only the genetic code and the invaded cell provides the raw material. The growth of viruses disrupts the invaded cell's internal mechanism and it usually dies and releases the viruses in the bloodstream. The invaded body's immune defenses react by producing an antiviral protein (interferon) which halts the spread of viruses to other cells. These defenses, however, sometimes are incapable of resisting the virus and are overwhelmed resulting in a diseased body. Antibiotics are ineffective against viruses and special types of drugs called vaccines (suspensions of attenuated or killed viruses) have to be developed for each type of virus. Out of some 5000 known species of viruses, over 200 have been identified as disease causing (pathogenic) to humans. New ones, like AIDS (Acquired Immune Deficiency Syndrome) in 1983 and SARS (Severe Acute Respiratory Syndrome) in 2003, appear suddenly and are often of animal origin. Some other common human diseases caused by viruses are chickenpox, common cold, herpes, influenza, mumps, measles, polio, rabies, rubella, and yellow fever. In 1971, microorganism smaller than viruses (called 'viroids') were discovered which can cause some rare diseases. In 1982, US neurologist Stanley Pruisner indicated the presence of a particle (called 'prion') about 1/100th the size of a virus and responsible for Bovine Spongiform Encephalopathy (BSE) and variant Creutzfeldt-Jakob Disease (vCJD) (see mad cow disease for details), for which he won the year 2000 Nobel Prize for physiology.
2. See computer virus.