In computer technology, viruses are malicious software programs, a form of malware. By definition, viruses exist on local disk and spread from one computer to another through sharing of “infected” files.
Using a floppy diskette from a machine that has a virus.
Downloading a file from the Internet (or bulletin board) that contains a virus.
Getting an email attachment that contains a virus.
Installing software purchased from a store or mail warehouse. While this is the rarest of the ways, software vendors have been found to ship their software with viruses (unknown to them of course) on the installations diskettes or cd-roms.