A small microorganism which cannot grow or reproduce apart from a living cell. A virus invades cells and uses their chemical machinery to keep itself alive and to replicate itself. It may reproduce with fidelity (identically the same) or with errors (mutations)—this ability to mutate is responsible for the ability of some viruses to change slightly in each infected person, making treatment more difficult. Viruses cause many common human infections, and are also responsible for a bevy of rare diseases.