Does the virus worm affect an I-MAC or just P.C.s?
There are several thousands known viruses and several hundred known
worms. I think you are asking about "Blaster".
This worm infects a computer using a well known security leak in
unpatched Windows NT, 2000, XP and 2003 systems.
There are other systems with a similiar security leak in the so called
"DCE (Distributed Computing Environment)" but there is no existing
virus/worm for those systems (until now). See http://www.kb.cert.org/vuls/id/377804