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OS X has got its own security systems, so you don't need any security app. If you want more information about OS X and viruses, see > http://www.thesafemac.com/mmg
Anyway, if you want an antivirus, install Sophos or ClamXav. Avoid the use of commercial security apps
Some use ClamXav (macupdate.com : https://www.macupdate.com/app/mac/15850/clamxav).
But mostly just do not be stupid: Do no go to untrusted websites, do not give the admin password to allow installation of software you did not search for explicitly, ...