(noun) any one of a group of substances that cause certain infectious diseases. Viruses are composed of protein and nucleic acid. They are smaller than ordinary bacteria and cannot be seen through microscopes. They are dependent upon living tissues for their reproduction and growth.

(noun) any program that modifies other legitimate programs, most often that not with malicious intentions, in such a way that the virulent program is run whenever the infected program is executed. They are dependent on files for their replication and survival.