It is possible that 600,000 Macs around the globe have been infected with a variation of the Flashback Trojan. Mallware Analyst Sorokin Ivan posted this information to his Twitter page late afternoon on April 4th 2012.
Oracle had released a fix for the vulnerability in February of this year, but Apple neglected to send out a patch until earlier this week, after the news hit.
What Should You Do If You Have a Mac?
First off you should make sure your Mac OS is up to date by running an update check. (see image below)
If you think one of your machines may be infected, F-Secure has instructions on how to use the Terminal to find out.