A new study by security researchers at Sophos Systems’ shows that Mac OS X isn’t as secure as many think, and may be a risk to Windows systems when used in the same environment.

Employing the metaphor of a sexually active individual carrying Chlamydia, Sophos’ Graham Cluely explains that 20% of all Macs in the study were carrying some form of Windows malware. This malware is no threat to the Mac, but can be passed onto Windows systems via a network, USB thumb drives, or e-mail attachments.

“Just like malware on your computer, Chlamydia commonly shows no obvious symptoms,” wrote Mr. Cluely. “But left undetected Chlamydia can cause serious problems.The good news is that Chlamydia is easy to treat. And, if it isn’t too tacky to make a parallel, so is malware on Macs.”

Sophos’ report shows that overall 2.7% of Macs were infected by some form of malware.

The most common malware infection was from the Flashback Trojan. Second to Flashback was the fake antivirus malware OSX/FakeAV, with 18% of the total.

“What Mac users really need to do is protect their computers now or risk allowing the malware problem on Macs to become as big as the problem on PCs in the future,” warned Mr. Cluely.