One in five Macs are carrying malware -- that is designed for Windows machines. That's a key finding of a new study of 100,000 Macs, which also found that 2.7 percent of Macs had malware designed for Apple's OS X platform.

The study by security firm Sophos was based on computers running its free anti-virus software. Graham Cluley, senior technology consultant at Sophos, said in a statement that "some Mac users may be relieved that they are seven times more likely to have Windows viruses, spyware and Trojans" than Mac ones, but "Mac users need a wake-up call about the growing malware problem."

Top Malware

Windows malware on a Mac cannot do harm on that platform, but it can cause damage if the user runs Windows as a secondary operating system, or if the infected files are shared with a Windows machine.

The top Mac OS X-based malware found by Sophos in a seven-day snapshot of the surveyed Macs were, in order of occurrence, OSX/Fishplyr, OSX/FakeAV, OSX/RSPlug and OSX/Jahlav.

Seventy-five percent of the Macs that had OS X malware had the Flashback Trojan, which Sophos refers to as OSX/Flshplyr. Late last week, the Russian security firm that first reported the Flashback malware said that the number of infected Macs was still about 650,000, and growing.

OSX/FakeAV is the name Sophos gives to attacking software that includes Mac Defender, which started infecting computers in spring of last year. Sophos said it was present on 18 percent of Macs with OS X malware.

Sophos said that Bredo malware, which accounts for 12.2 percent of malware on the surveyed Macs, is usually sent via spam, and was first detected in 2009.

The top Windows malware were Mal/Bredo, Mal/Phish, Mal/FakeAV, Troj/ObfJS, Mal/ASFDldr, Troj/Invo, Troj/Wimad, Mal/Iframe, and Mal/JavaGen.

Macs a 'Soft Target'

The company noted that Mac malware is spread by USB stick, e-mail attachments, downloads from Web sites, or a "silent drive-by installation where the user doesn't realize their Mac's security has been subverted" by visiting a Web site.

The security firm said that, while some Mac users are beginning to realize that their platform is increasingly being targeted, cyber-criminals look at Macs as a "soft target," because many owners don't have anti-virus software and often have higher levels of disposable income than typical Windows users.

In a Tuesday posting on his company's blog, Cluley made the interesting comparison that the percentage of Macs with Windows malware was approximately the same as the percentage of young people estimated to have the sexually transmitted disease, Chlamydia.

"If it isn't too tacky to make a parallel," he wrote, both are easy to treat.

For the software problem, Sophos recommends an up-to-date anti-virus program, up-to-date OS and application security patches, and caution about which programs are installed, which links are clicked, and which attachments are open.

Another security firm, Kaspersky Labs, recommends creating a non-administrator account for everyday activities, using a sandboxed-Web browser such as Chrome, uninstalling the standalone Flash Player, uninstalling Java or at least disabling it in browsers, and using Office 2011 instead of 2008.

Kaspersky also recommends using Mac's built-in Keychain password manager, enabling full disk encryption or FileVault, and upgrading to Adobe Reader 10 or later.