Back in the day, one of the more compelling reasons to buy a computer from the folks at Apple was that since there weren't as many of them out there, they were inherently safer: why would some nefarious hacker create an evil virus that targeted somewhere under five percent of all the machines sitting on desktops, when there was a world's worth of Windows boxes to go after. Over time, though, the installed base of various flavors of Macs has increased, and the day will come, if it hasn't already, when somebody's going to decide it's worth their while to put some nasty virus, Trojan, or worm out there that's going to eat your shiny new MacBook for lunch. And that's not good.
Sophos has a free antivirus solution for your Mac. It'll protect you from bad things that land on your machine, as well as Windows files that you may be handling in transit from here to there. It is updated constantly, so you're not protecting against just the last virus out there, but anticipating the next one as well, and since it monitors behavior as well as signatures, it can detect so-called zero day exploits even before they have a name, as well as dealing with all the oldies-but-goodies flying around out there.
Sophos Anti-Virus for Mac runs under OS X version 10.4 and later.