Mac users have long taken pride in the fact that there are very few viruses, Trojan horses, and other malware that exploit the Mac operating system. Apple has even used this fact quite effectively in television commercials, which contrast the greater security of Macs as compared to Windows PCs.

But even though the Mac operating system is considered far less vulnerable to attack than PCs, rarely, a virus will appear that specifically targets Macs. A new Trojan-type computer virus is making the rounds online, and is designed to exploit a vulnerability in the Mac OS X operating system.

Technically, the Trojan is known as AppleScript-THT, and is classified as a Trojan horse exploit. The virus affects the Apple Remote Desktop Agent software in OS X to compromise the operating system, and gives remote control of the user’s computer to hackers. The virus is transmitted online, and targets only Mac OS X operating systems, while ignoring Windows users completely (a nice change of pace).

In some ways though, the new Mac Trojan horse only serves as an example of how rare viruses are for Apple operating systems. Windows viruses are rarely considered news; at any given moment there are dozens of new viruses and malware which target PCs being tracked by security experts. In fact, an entire industry of PC security has been built on back of Windows vulnerabilities. Companies like McAfee, Norton and others sell millions of copies of their PC virus protection software.

But to date, Mac users have had a far easier time. And when a new Mac virus does appear, it quickly becomes big news, as there are so few of them to begin with. But the new THT Trojan demonstrates very clearly that even Mac computers are vulnerable to computer viruses and hackers.

Several different variations of the new Mac Trojan have been spotted online, most emanating from a hacker web site. There is even some speculation that the Trojan could be transmitted via Apple’s iChat instant messaging service. The virus has also been spotted on file sharing software such as Limewire.

The full virus script is bundled into an application called “ASthtv05,” and the file size is approximately 3 MB. Experts say that the virus can only be activated by opening up the program — inadvertently downloading it to your Mac is not enough. Because of this, it is unlikely that the Trojan will become a widespread problem for Mac users.

Some hard-core Mac users argue that the new Trojan is not actually a computer virus at all, but only an exploit, because a user must specifically download and open the file in order to become infected. Experts point out that viruses for Windows PCs are usually much more advanced, and require virtually no participation from the user to become active.

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