Security researchers said the first virus to strike Apple computers running the Mac OS X operating system discovered Thursday doesn’t cause any damage, even though it can be annoying because it can potentially spread to hundreds of machines.

The virus named “Leap A” or “Oompa” spreads through the iChat instant messaging systems on Apple’s Mac computers by forwarding itself as a file to contacts on the infected user’s buddy list. If the malicious file is clicked on and unzipped, the virus tries to spread to other contacts on the user’s buddy list.

The virus has been categorized as a Trojan horse because of the way it disguises itself in order to infect users.

“It is malicious in the way it tries to self propagate but it is not doing any damage,” said Fred Avolio, director of risk intelligence for Cybertrust, a Herndon, Virginia-based security services firm. “It is nasty because it exhibits unsavory behavior but does not do much beyond that.”

Cybertrust believes the threat from the Oompa Trojan is low because it does not cause any harm and the cost of fixing the threat for businesses or users is low. The virus is considered a “proof of concept” and was probably written by virus writers to show that a vulnerability exists in the Mac OS X and can be exploited, experts said.

Still, the Oompa Trojan is expected to generate immense interest among Mac users because attacks on Mac are scarce.

“We suspect that it is going to make a big splash,” said Mr. Avolio. “Mac users like to point out that there are very few viruses for Macs and Unix systems.”

First Ever for Mac OS X

The Oompa virus is the first for the Mac OS X platform but other viruses and worms for the Mac computer have been discovered.

However, Mac has been a much safer operating system compared to Microsoft’s Windows OS, and security has often been a strong selling point with Mac users.

“Mac viruses are kind of rare,” said Shane Coursen, senior technical consultant with Kaspersky Lab, an antivirus specialist. “We don’t see a lot of people writing viruses for the Mac because the number of Mac users is not as great as Windows.”

Mr. Coursen said he estimates there are about 200 viruses for the Mac compared to more than 150,000 viruses for the PC.

Mac Under Attack

Security experts said the Oompa Trojan is unlikely to plague many users, but future viruses or worms for the Mac could have a greater impact.

As Apple introduces its Intel-chip-based Macs, there will be greater adoption of the operating system, which could trigger malware writers into creating threats for the Mac, Mr. Coursen said.

Other experts said the latest virus could act as a reminder for Mac users that their machines are susceptible to security threats, much like the machines of PC users.

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