Don't you use anti-virus on your Mac? I know that Linux-based systems are significantly less likely to be attacked by viruses, but I assumed Macs - being as popular as they are - would be fair game for malware writers.

With the exception of net-side threats like worms I don't think anti-virus software is needed at all if you're half-way sentient about what you're doing/installing/downloading. I'm not, unfortunately. I once downloaded an .exe which I thought was a list of recipes for a bread machine. Completely fucking destroyed my system. What a twat.