I guess it can happen to anyone. This weekend I started getting emails from friends and automated rejections form mail servers. It seems that my Yahoo account got hit with a virus that sends spam from my mail account to everyone in my address book. Now the surprising part is that this happened while running a web mail browser session which did not give an indication that a message was sent either visually or leaving a message in my sent mail. It also occurred without setting off warnings from either my local virus software (which is automatically updated) or any software that may or may no exist on Yahoo's servers.
So what can be learned from this experience. Nothing is ever 100% safe. We need to improve our detection systems to keep this sort of incident to a minimum. I know there has been talk of software that will search for virus patterns instead of specific virus signatures.
The whole experience left me feeling vulnerable. As a result I ended up checking all of my online accounts and changing passwords to make sure that nothing else was compromised.
Stay safe (or at least try).