All Mac users should quickly check Software Update (click on the Apple Logo at the top left hand side of the screen) in order to download the Flashback removal tool. Flashback is the Java security breach that affected over 500,000 Macs. Users visiting insecure websites have been infected by the trojan which then compromises the host computers.
This Java security update removes the most common variants of the Flashback malware.
This update also configures the Java web plug-in to disable the automatic execution of Java applets. Users may re-enable automatic execution of Java applets using the Java Preferences application. If the Java web plug-in detects that no applets have been run for an extended period of time it will again disable Java applets.
This update is recommended for all Mac users with Java installed.
For details about this update see: http://support.apple.com/kb/HT5242