They've existed for a long time, and made a bit more headlines recently so people are getting more excited, but if you use common sense (don't run software if you don't know where it's from, backup your data in at least 2 places, etc), have a secure browser (like Chrome), and install patches, you're chances of getting a Mac virus are so low that the assured inconveniences that come from an anti-virus are MASSIVELY bigger than any inconvenience you could get from a virus. Not worth it (maybe some day it will, but the UNIX foundations of OS X make it pretty secure by default).