The word virus means "poison". It was thought at first than an infectious and poisonous fluid was responsible for viral diseases. Viruses inject living animals and plants and grow and multiply in them. In many they cause diseases like mumps, small pox, chicken pox, measles, common cold, yellow fever, polio, cancer, influenza, hepatitis and jaundice. They also cause brain diseases like paralysis and rabies.

They cause diseases in plants like the potato, tomato, tobacco, gourds, cucumber, cabbage, turnip, bananas, sugarcane and peach. Virus even effect bacteria. There is one virus known as the bacteriophage which eats up bacteria and thus is helpful to man.

Though many vaccines have been developed to build artificial immunity against viral disease, scientists still have a long way to go in the battle against the dreaded virus.