It really isn't "much harder" to get a virus on a computer running Mac OS. Before the Windows revolution, Mac viruses were everywhere. The only reason they practically stopped happening is because of the shrinking popularity over the years. Now, with the large number of Macs being sold, the number of Mac viruses has gone back up. Just like Linux, you can get a virus. The only way it's "harder" to get one is the programmer needs to work harder at either tricking you into activating it, or bypassing certain security measures that limit the infection by OS design; that's about it.
According to AV-Test, the best free solution is Avast.
According to the AV-Comparatives 2014 report, Avast would also come out as the overall free...