In an advisory, Sophos said its experts at SophosLabs, the company’s global network of virus, spyware and spam analysis centers, have discovered the virus named Leap-A, which was found spreading via the iChat instant messaging system.

Sophos said the worm forwards itself as a file called "latestpics.tgz" to contacts on the infected users’ buddy list.

When the latestpics.tgz file is opened on a computer, it disguises itself with a JPEG graphic icon in an attempt to fool people into thinking it is harmless, Sophos said.

The worm uses the text "oompa" as an infection marker in the resource forks of infected programs to prevent it from re-infecting the same files.

"Some owners of Mac computers have held the belief that Mac OS X is incapable of harboring computer viruses, but Leap-A will leave them shellshocked, as it shows that the malware threat on Mac OS X is real," said Graham Cluley, senior technology consultant for Sophos. "Mac users shouldn’t think it’s okay to lie back and not worry about viruses."

The executive said Apple Mac users need to be just as careful running unknown or unsolicited code on their computers as their friends and colleagues running Windows.

Sophos advises all computer users, whether running PCs or Macs, to practice safe computing and keep their anti-virus software updated.

User of the Mac platform have been generally unaffected by Internet worms, because these malicious programs usually targets and exploits Windows-based systems.