While fewer Macs get viruses than PCs, Apple computers are increasingly being used to spread them across the internet, with one in five carrying some form of malware.

Internet security expert Sophos has revealed that after analysing a sample batch of 100,000 Macs, 20 percent were carrying one or more instances of Windows malware.

The Windows virus will not harm the Mac - unless it's running a Windows operating system, of course - but the Mac will happily pass the virus on to any unprotected Windows machine that it comes into contact with.

Worse still, Sophos's research found that of the 100,000 Macs that had recently downloaded the firm's anti-virus software, 2.7 percent (one in 37) were infected with Mac OS X malware.

Security expert Graham Cluley said on his blog: "Fake anti-virus attacks, which scare users into handing over their credit card details or installing unsafe software, and the recent 600,000 strong Flashback botnet (called Flshplyr by Sophos security products) dominate the chart [below] of Mac-based threats."

Mac viruses
Mac OS X infection are dominated by the Flashback bot net and fake antivirus software. (credit: Sophos)

While Mac viruses are far rarer than their PC equivalents - and Mac users will be happy to see that they are seven times more likely to be carrying harmless PC malware than anything damaging to OS X - the Mac is seen as a soft target.

Cluley continues: "Sadly, cybercriminals view Macs as a soft target, because their owners are less likely to be running anti-virus software. Bad guys may also believe that Mac users are likely to have a high level of disposable income than the typical Windows user.

"So, they might believe the potential for return [from paying out for fake anti-virus software] is much higher."

The graph below shows the most popular Windows malware found on Mac computers:

Sophos graph
(credit: Sophos)

Mac users' complacency and belief that their computers will not get infected has resulted in Macs carrying viruses that are several years old, and could have easily been removed.

"Amazingly, some of the malware discovered by Sophos on the 100,000 Mac computers sampled dates back to 2007, and would have been easily detected if the users had run an antivirus sooner," Cluley said.