New research from the security firm Sophos draws attention to a malware threat that likely never crosses most people’s minds: The study asserts that one in every five Macs has Windows malware, which can be passed on to other computers.

In fact, Mac users are seven times more likely to have Windows malware on their computers than Mac malware. According to the study, only one in 36 Mac users were found to be carrying viruses, spyware or Trojans intended for Mac OS X. But even if Mac users don’t see the bad effects of a Windows virus, they risk passing on the malware to Windows users through shared documents, thumb drives, e-mails and other means, which is probably how Mac users acquire Windows viruses in the first place.

Sophos conducted the study by looking at data gathered from 100,000 users of its own Mac antivirus program. Some of the malware found dated back to 2007, the company said, indicating that many Mac users rarely, if ever, run regular scans for bad software.

And for many years, it seemed they didn’t have to run scans. But as Macs gain market share, more thieves are targeting that base, the security firm said.

“Cybercriminals view Macs as a soft target, because their owners don't typically run anti-virus software and are thought to have a higher level of disposable income than the typical Windows user,” said Sophos consultant Graham Cluley in a release. “Mac users must protect their computers now or risk making the malware problem on Macs as big as the problem on PCs.”

One Mac trojan that has been in the news lately, of course, is the Flashback trojan. Apple itself acknowledged the bug, releasing an update intended to remove most variants of the malware. Yet while it appeared that the Apple fix was reducing the number of computers affected, the Russian antivirus firm that first publicized the attack, Dr. Web, told Computerworld’s Gregg Keizer that the Flashback trojan is still on around 650,000 Macs and growing. On Friday, Keizer reported that figure was backed up by fellow security firm Symantec, which had earlier reported that the Flashback botnet had been reduced by 60 percent, the report said.

Macs still do not face the same threat from malware that Windows computers do, but the Sophos study shows that most Mac users may not be paying as much attention to threats as they should, either. There’s no need to panic about an incoming flood of Mac malware, but it’s just common sense to check for threats — particularly if your computer starts to act strangely. There are a couple of free, basic programs out there in addition to Sophos free Mac antivirus, including the open source download ClamXav and iAntiVirus as well as paid offerings from companies that make PC and Mac antivirus software.

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