More than half a million Apple computers have been infected with the Flashback Trojan, according to a Russian anti-virus firm.
Its report claims that about 600,000 Macs have installed the malware - potentially allowing them to be hijacked and used as a "botnet".
The firm, Dr Web, says that more than half that number are based in the US.
Apple has released a security update, but users who have not installed the patch remain exposed.
Flashback was first detected last September when anti-virus researchers flagged up software masquerading itself as a Flash Player update. Once downloaded it deactivated some of the computer's security software.
Source: http://www.bbc.co.uk/news/science-environment-17623422
