What you need to know about the Flashback trojan
It’s rare that any sort of viruses effect Mac computers but this one is legit and it’s spreading. The trojan exploits an issue with Java (which most people I’m sure have no clue what that is) which is a plugin (like Adobe Flash) that you really don’t need. So at a minimum, follow the instructions to make sure you have the latest patch (just run System Update from your Mac’s Apple menu), a better recommendation though is to do what I did and follow the instructions to just disable Java altogether.