Stop, drop, and uninstall.
Russian anti-virus vendor Dr. Web recently revealed that over 600,000 Macs were infected by the trojan BackDoor.Flashback this past week. The original figure was quoted at 550,000 but Dr. Web analyst Sorokin Ivan later confirmed via Twitter that the figure had risen to over 600k; 274 of those infected Macs were from Cupertino, California.
The infection was originally reported back in September 2011 and used an exploit in Adobe Flash Player to install itself onto a PC, but in recent weeks it has re-surfaced via loopholes in some Java vulnerabilities. Apple has already released a patch to address this issue and is urging Mac users to apply this update as soon as possible.
If a system is compromised and the Trojan gets installed, it then injects code into web browsers and other applications to try to harvest passwords and other sensitive data. Instructions on removing the trojan can be found here.