The Flashback botnet scare may have thrust Macs' supposed invulnerability to antiviruses claim under a microscope, but Sophos decided it wanted some numbers to go along with the heaping of hype. So the company studied feedback from 100,000 Apple computers with Sophos antivirus installed and surprisingly discovered that the Macs were fairly teeming with malware. Before you start laughing, consider this: the vast majority of the malware found didn't affect OS X at all. It targeted Windows PCs.
Only 2.7 percent of the infected Macs contained malware that was harmful to Apple computers, Sophos reports in both a press release and a post on its Naked Security blog. However, a whopping 20 percent of Macs -- that's one in five, if math isn't your strong point -- were riddled with "one or more instances of Windows malware." Most of the Mac-targeting bugs were either Flashback or fake antivirus scams, while the top Windows malware found on Macs turns PCs into spam factories.
Yes, Macs can often transmit that malware to PCs.
Sophos says that some of the PC malware infected the Macs as far back as 2007 and could have been easily removed at any point if the hardware owners would have installed an antivirus program rather than buying into the whole "Macs don't get viruses" thing. Sophos' Graham Cluley also says PC malware on Macs is a lot like Chlamydia:
Just like malware on your computer, Chlamydia commonly shows no obvious symptoms. But left undetected Chlamydia can cause serious problems, such as infertility… The good news is that Chlamydia is easy to treat. And, if it isn't too tacky to make a parallel, so is malware on Macs.
Cluley then went on to plug Sophos' free antivirus product for Mac users. Keep that in mind while you're contemplating these numbers. The study also drew its sample from 100,000 Macs that "recently" installed Sophos antivirus, which means the stats could be skewed somewhat, as you aren't likely to install a new antivirus program unless you're worried that either a) you're Mac is infected or b) the Flashback boogieman is going to get you.