Sophos analyzed around 100,000 Mac computers running its anti-virus software and discovered that 20% of Macs were carrying one or more instances of Windows malware.
The company noted that while Windows malware will not cause symptoms on the Mac, it can still spread to other computers.
The analysis also found that 2.7% of Macs were carrying Mac OS X malware. The top Mac OS X malware found on the infected machines were OSX/Flshplyr (75.1%), OSX/FakeAV (17.8%), OSX/RSPlug (5.5%), OSX/Jahlav (1.2%), and other (0.4%).
"Some Mac users may be relieved that they are seven times more likely to have Windows viruses, spyware, and trojans on their Macs than Mac OS X-specific malware, but Mac malware is being surprisingly commonly encountered. Mac users need a loud wake-up call about the growing malware problem", wrote Graham Cluley, senior technology consultant at Sophos, in a blog.
“Mac malware can spread via USB stick, email attachments, website download, or even a silent drive-by installation where the user doesn't realize their Mac's security has been subverted. 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. Mac users must protect their computers now or risk making the malware problem on Macs as big as the problem on PCs", Cluley added.