Get this, then get ClamXav as a back–up.

Contrary to popular belief, OS X is no longer virus–proof (if it ever really was). There have been a number of cross–platform viruses recently (mostly exploiting Java) that have been hitting users of traditionally ‘safe’ operating systems like OS X and Linux. Dr Web Light, along with ClamXav, are the best anti–virus tools Ive found for OS X. Dr Webs database is typically more current, so it is more likely to catch things ClamXav has missed. I have rooted out infections in the HDs of a few different Macs by this point— and none of them belonged to users prone to risky activity. While you may not feel like you need it, its good to have around. At least one of the current bugs was preventing users from searching for anti–virus programs on Google, among other things.

Aristarch Jones about Dr.Web Light, v6.0.6