Malware on the Mac has been a hot topic in recent weeks, thanks to the discovery at the beginning of the month of a variant of the Flashback botnet that was able to install on users’ computers without their knowledge by using a Java exploit. Though a Java patch and a removal tool have been released and infection rates are going down, Mac-based malware remains on many users’ minds.

Macs have long been thought to be virtually immune to malware. That, however, has proven untrue. The fact is that although the underlying structure of the Mac OS is somewhat more secure than that of Windows, the main thing that has been protecting Mac users for all these years is the Mac’s far lower market share. Since the vast majority of personal computers are Windows-based, malware creators see little need to focus their attention on Macs. Flashback has proven, however, that Macs are much more vulnerable than Mac users had previously thought.

Now, a recent report has shown that far more Macs are infected with malware than previously thought. Sophos Security, makers of free anti-virus software for Macs, has released a report showing that 20% of Macs are carrying around malware. Now, although that number seems awfully high, there is a bright side: the malware those Macs are infected with is actually designed for Windows computers. That’s good news for Mac users, as the malware isn’t likely to actually harm their computers. The bad news, however, is that if you’re a Mac user whose computer is infected with Windows malware, you’re a carrier. Unless your computer only interacts with other Macs, then you run the risk of infecting Windows computers with malware from your Mac.

When it comes to malware that’s actually targeted at Macs, the picture is a little less grim. Sophos’s data shows that only 1 in 36 Macs is infected with Malware actually designed for Macs (like Flashback). Unfortunately, though, the odds are pretty good that that number will go up in the not-too-distant future. As Sophos points out, Macs make a “soft target” for hackers, because the majority of Mac users don’t have anti-malware software installed on their machines. After all, it’s a Mac, right? Sadly, the days when Mac users could hide behind their computer’s brand name appear to coming to an end.

Fortunately for Mac users, though, there is a decent selection of free antivirus software available for Macs. In addition to Sophos’s own software, there is ClamXav, which has been around since long before most Mac users thought they needed anti-virus software, as well as PC Tools iAntivirus. There are also paid options offered by the “big names” in computer security – McAfee and Norton.

Mac users, do you use anti-virus software on your Mac? If not, do you plan to install it anytime soon? Let us know in the comments.