The argument that one should run anti-virus software on a Mac to protect against passing virus-infected files to Windows users is flawed, in many ways:
- You can't create a file on Mac OS X that contains any Windows malware. The only way you can have such a file on your Mac is to first get it from a Windows computer.
- If you are downloading files from less than reputable sites (which includes file sharing, porn and gaming sites), you should never send such files to anyone, regardless of the OS they're using.
- Before you receive files from any Windows user, you should be prudent enough to make sure they're running AV on their computer.
- Before you send files to any Windows user, you should be prudent enough to make sure they're running AV on their computer.
- You should never forward an email to anyone if it contains attachments that you're not certain are safe or came from a safe source.
- If you run AV on your Mac but fail to make sure your Windows-using friends are also running AV, you have failed to help them, as you have left them vulnerable to much greater malware threats from sources other than your Mac.
Protection against malware is the responsibility of each and every computer user. That responsibility cannot be effectively transferred to anyone else.