My mac had a worm virus - nearly drove me mad...It took a life of its own on, doing odd things, and quitting programs on its own.
The final straw came when a little alien walked on screen, calmly walked across my QuarkXpress document. (Which was a huge project I was working on for a client) the little divil then proceeded to zap with his ray gun all my work.
So, there you go, it does happen. But not often.
Ella