Generally, you don’t hear much about viruses and other malware effecting Macs but the Flash Back Trojan seems to be one of the big exceptions. Over the last week it was discovered that over 600K Macs have been infected, with 50% of those being in the US and 20% in Canada. The exploit is so widespread it is even said that 24 of those were actually on Apple’s Cupertino campus.The trojan works via an exploit in java, and is triggered by clicking a web link. It’s said that it was being used to perpetrate click fraud, and that it is no longer an issue but one can never be too safe. For that reason, F-secure has a guide to help you determine if your machine is infected and if so, what you can do about it. Also worth noting is that by default in OSX Lion Java is not pre-installed. If your computer shipped with Lion unless you specifically installed java you know you are not infected. The guide can be found here.