About 20 percent of all Macs are infected with some sort of malware and most of this malware were intended to Windows machines. A variation of the Windows classic Flashback Trojan has recently made the news after it infected over 500,000 Mac machines worldwide.
Security firm Sophos analyzed over 100,000 Macs to find out that 20 percent of them were carrying out viruses intended for Windows machines. Only 2.7 percent of them had any kind of Mac-intended infection, which leads us to believe that Macs can be a huge spreader of Windows malwares.
Graham Cluley, a Sophos analyst, said that most (if not all) of these viruses which were created for Windows machines do not harm Mac OS X at all unless the person is running two operating systems in just one hard drive; Macs then, become spreaders of Windows malware.
"[Windows] Malware can spread onto Macs via USB drives, email attachments, website download, or even a silent drive-by installation where the user doesn't realize their Mac's security has been subverted," Cluley said; and most of these infections could have been easily avoided if Mac users used anti-virus software. Over 75 percent of them believe that using any kind of anti-virus is completely unnecessary.