It is sad to say, butMac has become a target for malware. Mac Defender proved that. So it’s time to start using an anti-virus. There are many out there. Sourcefire has ClamAV which Mark Allen has put a nice front end on to create ClamXav (open source and free, but please donate) which works well on a Mac. Another good option is a slightly more user friendly Sophos http://www.sophos.com/en-us/products/free-tools/sophos-antivirus-for-mac-home-edition.aspx (again free). I’ve tried using Norton and McAfee in the past but they were resource hogs. I don’t think it matters as much what you use, just that you are using one.
Little Snitch is nice too, it’s not an anti-virus but it does alert you to all the apps that are talking across your network. Interesting to see if nothing else.
No script is another tool to look at if you use Firefox (or not scripts for Chrome). Again, I allow most scripts to run, but it is good to see what scripts different websites are running and what they are tracking.