Long immune from the viruses that plague the Windows world, Mac users now face their first significant threat in the form of OSX/MacDefender.A.
The virus presents itself as a link in search results which, when clicked, according to PCWorld, "launches a Java-based scan script that claims to find malware on the user's system".
The site reports: "If the victim falls for this ruse, the malware starts to install a convincing-looking but bogus application called MAC Defender, which also requests the admin password. The application will then launch itself every time the Mac is turned on and makes it as difficult to stop running as the Dock function has been disabled."
Read more here.
Macworld has fairly recent information on free antivirus apps for Mac.