Read more at: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.
Maximum PC | Sophos: 20 Percent Of Macs Hide A Chlamydia-Like Risk For Windows PCs