Half a million Mac computers have been infected with malware
Over half a million Apple computers have been infected with a trojan called the Flashback Trojan. A report from a Russian anti-virus firm says that about 600,000 Macs have the malware installed which puts them at risk for the crippled computer to send a message to the intruder's control server with an ID that is specific to each computer.
These intruders could then have the possibility of remotely controlling your machine. Most of the infected computers are located in the US. Apple has released a fix for this loophole, but unless you've updated you'll be at risk.