A virus is smaller than the smallest bacteria visible through an ordinary microscope. Viruses can multiply only within the living cells of humans, animals or plants. They are responsible for a wide range of infectious diseases and illnesses.
Viruses can be investigated with the help of the electron microscope, and their measurements recorded. The virus responsible for foot and mouth diseases in animals measures about one-millionth of an inch. The smallpox virus is about ten times as large.
Diseases caused by viruses very greatly in danger and intensity from minor infections such as clods to extremely serious epidemic diseases. Either way, it is well worth taking careful precautions against catching a virus.