This past week Apple added a patch to a Java vulnerability as part of a software update. The vulnerability allowed a virus called the Flashback trojan to infect about 550,000 Macs. If you want to find out whether your Mac was infected, there is a simple way to find out. Simply download the zip file from this website, unzip it, and open up the two files. I have tried it myself and it works fine. The message that you should be hoping to see when opening the files should be something like this:
If your Mac was infected, there are removal instructions over at F-Secure.com.