Mac owners have become comfortable with the idea that virus issues which plague PC / Windows users are not their problem. But - sometimes they are a problem for Mac users as well.
Consider the case where a PC user sends to your Mac a Word document infected with a virus. The virus will have no effect on you - it is designed to attack only a PC. You, however, can inadvertently circulate the virus farther when you send the file along to your colleagues or friends. Virus protection on your Mac would catch PC viruses as well.
Sophos has made available a free anti-virus tool for Mac home users: Sophos Anti-Virus for Mac Home Edition. You can download it here.