half the computers infected with malware designed to steal personal information are in the U.S.
As CNET blogger Topher Kessler explains, simply visiting a malicious Web site containing Flashback on an OS X system with Java installed will result in one of two installation routes. The malware will request an administrator password, and if one is supplied, it will install its package of code into the Applications folder. If a password is not offered, the malware will install to the user accounts where it can run in a more global manner.
Instructions for Detection and removal here