Although the Windows malware is not harming the Macs, because they use different operating systems, the computers are spreading it to other machines.
The study also found that 2.7 per cent of Macs carry their own, harmful Mac malware.
Apple computers have long been considered invulnerable to viruses, but the Flashback virus, or botnet, was recently discovered on more than 600,000 Macs. Apple released a patch to solve the problem, which brought the number of infections down rapidly.
Sophos analysed approximately 100,000 Mac computers running its free anti-virus software.
Graham Cluley, senior technology consultant at Sophos, 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. Mac users must protect their computers now or risk making the malware problem on Macs as big as the problem on PCs.
"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."
Sophos said fake anti-virus attacks, which scare users into handing over their credit card details, and Flashback, dominate the chart of Mac-based threats.
Cluley claimed "Mac malware can spread via USB stick, email attachments, website download, or even a silent drive-by installation where the user doesn't realise their Mac's security has been subverted."
Some malware on Macs dates back as far as 2007, and the most common, the Bredo-virus, was first found in 2009.
Researchers at Intego recently reported that although the Flashback virus was diminishing, a new variant, Flashback.S was now spreading itself actively.
