More than half a million Apple computers have been infected with the Flashback Trojan, according to a Russian anti-virus firm.
It suggests that about 600,000 Macs have installed the malware - potentially allowing them to be hijacked and used as a "botnet".
The company says that more than half that number are based in the United States.
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 software masquerading itself as a Flash Player update.
