So far, the so-called "flashback trojan" malware has infected more than 600,000 Mac computers, most of them in the United States and Canada.
The flashback virus was first discovered in September 2011. It was initially designed by hackers to resemble an Adobe Flash Player installer, but would seek out user names and passwords stored on Macs. Later versions exploited weaknesses in Java programming language and allowed the virus to be installed from websites. CNET blogger Topher Kessler wrote that flashback can inject code into Web browsers and other applications like Skype to harvest passwords and other information.
Apple released a new security update Wednesday to protect Mac users against the virus.
Additionally, security company F-Secure has published instructions on how to determine whether a Mac is infected with flashback.