Virus an ultramicroscopic infectious agent that reproduces only in living cells; usually considered as lying in the borderline between the living and the non-living world made up of a protein coat (capsid) inside which the genetic material, DNA or RNA, is housed. Because the virus cannot multiply except when inside another living cell, it can be called a perpetual parasite. There are bacterial viruses, the bacteriophage, which grow in bacteria. Then there are plant viruses and animal viruses. Some common viral diseases of man are: AIDS, yellow fever, dengue fever, poliomyelitis, hepatitis, rabies, influenza, mumps, measles, pox, herpes, rotavirus diarrhoea, Chron's disease, and a variety of encephalitis. Several types of cancers are believed to be caused by a class of virus called oncogenic virus. Many plant viruses cause disease in agriculturally important plants such as rice, tobacco, tomato, potato, cauliflower, and many fruits. [Zia Uddin Ahmed]