Yes, it is rare. Most Mac users work with the relatively safe feeling that Macs don't get viruses. Don't get lulled into a false sense of security. With the surging popularity of all things Mac, your trusty and reliable Mac is now a target more than ever. And that means you are too!
CNET reports that over 600,000, almost 60% of Mac users in the U.S. alone, are infected with the Flashback botnet/Trojan that was designed to steal sensitive information and send it to identity thieves. The Trojan originally came packaged as a wrongly named Adobe Flash Player plug-in installer back in September. The new tactic, Flashback.N is a modified version that searches for Java vulnerabilities and installs itself when you visit an infected web site. If you have visited one of those sites your computer is at risk. It appears that Flashback is using Twitter to deliver commands and directions.
Apple issued an update for its OS X operating system to patch the problem on April 4. If you want to be sure that you AREN'T infected, there is a manual check that you can perform using Terminal and simple cut-and-paste to see if you are infected.