OS X has got its own security systems installed, like XProtect (it's a file with virus definitions that checks your downloaded files) and Gatekeeper (it prevents you to install applications from unknown sources, although you can turn it off). Also, Windows viruses don't affect OS X.
If you want more information about OS X and malware, see > http://www.reedcorner.net/mmg
Yes it does, but:
You may find this User Tip on Viruses, Trojan Detection and Removal, as well as general Internet Security and Privacy, useful: The User Tip seeks to offer guidance on the main security threats and how to avoid them.
More useful information can also be found here: