As the popularity of Macs increases, it's inevitable that they will become the target of viruses and although the risk is still small, you haven't got much to lose by installing a free virus checker like ClamXav.
ClamXav is a free virus checker which can perform full system scans or analyze specific folders. The latest release has seen a much improved interface mainly due to the move away from Java and to Objective-C. Installation of ClamXav is a bit lengthy as it also requires installing the ClamAV engine which also requires a script (included in the installation file) to remove it.
The interface of ClamXav is simple and straightforward. You can schedule automatic system scans to be performed regularly, by setting the day of the week and hour at which you want ClamXav to start its analysis. It's obviously best to schedule full scans while you're not using your Mac because as with most virus checker, it can slow down your system somewhat. In fact, scanning in general is very slow with ClamXav although for checking individual files it's fine. Probably the best way to use ClamXav is via the "Sentry" which makes ClamXav monitor for changes in individual files and folders rather than performing slow regular system scans.
If you use it sparingly, ClamXav is a pretty good free virus checker with a regularly updated database although speed is not its forte.
- Fix for ClamXav Sentry crashes. Fix for Sentry not logging to the right place.