A type of intrusive malware that replicates itself and inserts copies of itself in legitimate programs, where it carries out unwanted and often damaging operations. Viruses initially were spread through infected floppy disks, which users frequently exchanged to share data and software. The most common contemporary methods of propagation are through attachments to Internet e-mail and programs downloaded from Websites. Viruses can be prevented if users open attachments only from trusted correspon dents, visit only trusted websites, and purchase anti-virus software that they keep current. The term virus, in the contemporary context, was first used by Fred Cohen in his paper "Experiments with Computer Viruses" (1984). According to Cohen, the term was coined by Len Adleman; however, the term was in common usage long before.The science fiction novel When HARLIE was One (1972), by David Gerrold, describes a computer program named VIRUS, which could be countered by a program named VACCINE. See also malware, spyware, Trojan horse, and worm.