Earlier this month Sophos released a free version of their product for the Mac, Sophos Anti-Virus for Mac Home Edition. Since then, they have been gathering data from their users. They recently released some statistics from the clients on their Naked Security Blog and found that users should be concerned about what might be hiding on their hard drive.
So far Sophos says that they have 150,000 Mac users and of those users nearly 50,000 have already reported finding malware on the computer. Interestingly enough the most common malware that the software detected was Mal/ASFDldr-A, a malware targeted at Windows machines. Mal/ASDFDldr-A takes advantage of a flaw in Windows Media Player which allows the infected media file to open a malicious web page rather than display the desired content.
They found though that the majority of attacks detected were Java exploits which are cross platform and were typically found in the internet cache on users computers. But lower on the list they did find some OS X specific pieces of malware OSX/Jahlav-C and OSX/DNSCha-E. OSX/Jahlav-C is a variant of OSX/DNSCha-E which happens to be a piece of malware that known as a DNS changer. When a DNS changer hijacks your computers DNS settings your web surfing can be almost completely compromised. Even if it looks like you are at Google.com and the page looks like Google's home page your DNS settings may have sent you to a server in China where malicious code is being executed in your browser.
The results show that the majority of files are more dangerous on Windows than on a Mac but that doesn't mean the virus goes away. If you plan to share files with a Windows computer it would be a good idea to maintain some sort of anti-virus protection just to be sure you are not spreading a virus to a more vulnerable operating system.