Venom means a poisonous substance. In old times people believed that some diseases are caused due to poisons produced in the body. Later it was shown that many diseases are caused by bacteria. But cause of some diseases still remained unknown. When chamberland showed that bacteria cannot cross porcelain filter but some agents can, people got more convinced that these filterable agents were liquid venoms. Later it was shown that these are actually viruses. The term virus is derived from Latin word for liquid poison / venom.