Malware hunters at Symantec have discovered a direct link between a malicious file embedded in pirated copies of Apple’s iWork 09 software and what appears to be the first Mac OS X botnet launching denial-of-service attacks.
iBotnet: Researchers find signs of zombie Macs | Zero Day | ZDNet.com
And that, boys and girls, is why you should not download pirated software. Because all the safe operating systems in the world can not be made immune against human stupidity. 
That’s why Trojan Horses are so successful. Just tag along in software and open the door to all sorts of bad things.
The question is why downloading some software, when the best software is free and opensource??? 
If everyone decides to use opensource software, how will these anti-virus companies make money?
They need proprietary systems with a lot of security holes to survive. They also want to make sure that the security issues should not get fixed fast enough so that hackers can exploit them.
>:)