uh, bad mistake.
virus is always man-made, although there are polymorph viruses which reprogram themselves in order to evade anti-virus-programs and the like. now these are nasty little fuckers, and their creators must be all the like.
then there's bugs which simply are errors in a program. they may cause system crashes etc. but don't usually harm other files except those they're working with (too bad if these are system files).
however, chain mails for example are viruses as well, but without a program! usually viruses spread themselves (via email etc), but in this case people are made to believe it would benefit them to send this mail to as many recipients as possible. so they manually spread the virus which isn't a virus after all... but it steals time from those who read it (@ work) and so costs an considerable amount of money...