1. Animal Diseases: Several of the animal viruses cause important diseases. Poliomyelitis cause by polio-virus was a wide spread, cropping disease through the first half of the twentieth century. Although poliomyelitis is now largely under control by vaccination in the industrialized countries, it remains a serious and common disease in the tropics and elsewhere in the less developed parts of the world. Colds are viral infection of the upper respiratory tract. About one third of all colds are caused by the rhino-viruses which are unenveloped plus-strand RNA viruses. There are dozens of different strains of clod-causing rhino-viruses alone, each of them with different properties and none conferring cross-immunity to the others. More than 200 of viruses that cause colds have been identified, which makes the development of appropriate immunization methods very difficult, if not impossible.
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