The "Klez" virus takes its mailing addresses (both the "send to" address, and a bogus "from" address) from the infected computer's "sent mail" file, and creates random messages.
Thus, computer A will send the virus to computer B, saying it is from computer C.
Owner of computer A won't know he's sending the virus to others.
Once on your computer, Klez disables your antivirus software (at least older versions). It seems to be changing nearly every day, probably hijacked by other hackers and rewritten to their own forms. It can be very difficult to remove.
vintage content - if I were ever to creat a virus, I think it would be pretty simple - Kambiokorsa, which requires that you press the "shift" key each time you start a new sentence.
Aldo Ross
Soggytown, Monroe, Ohio