You can NOT say something like "Macs do not need anti-virus software" without knowing anything about the situation. It's already been mentioned that Macs can host viruses of any kind and can pass them along to Windows computers, making virus software on your Mac quite useful and relevant, especially if you share files with Windows users. Furthermore if you run Windows "on your Mac" then you will need virus software.
That is antivirus software to protect Windows. OS X doesn't need it, unless you are trying to protected Windows.
"Macs do not need antivirus software" is an incomplete statement at best, comically wrong at worst.
To protect OS X, it isn't needed.