Russian security firm Dr.Web claims over 600,000 Mac OS X computers are infected by "Backdoor.Flashback", a Trojan horse that installs itself via a Java-applet.
Malware for OS X, called "Backdoor.Flashback", is running on up to 550,000 different machines mostly located in the United States and Canada. Dr.Web's report is extremely detailed, including an infographic of infections by countries. The botnet stems from machines being redirected to bogus websites, or other traffic distribution systems. Sites used for this are presumably of Russian origin, but the number of sites is currently unknown.
Exploits are being distributed over three main weaknesses:
Dr.Web initially reported that 550,000 Macs are infected but revised the number to "over 600,000" via a Twitter message.