I'm fairly sure someone's going to wade into the argument to tell you the sky will fall so I'll offer my point of view.
MAC OS virus' do exist but as long as you adopt the same common sense approach as PC users should when opening files from external sources, email attachments or p2p for example, then the chances of you encountering one are minuscule. The argument likely will come up that if you're networked to PC's then you should use an antivirus but again, safe practice is really the only way to prevent infection, don't download windows executables or batch files.