For a long time, email was the primary vector for viruses, before that floppy discs carried bugs from pc to pc. Then came network worms exploiting windows security vulnerabilities which led to the rise of firewalls and the increase in viruses piggy-backing into the system through browser bugs. But, any program that listens for data coming from the network could be an entry way for good traffic, or bad. The Securityfix is talking today about November being a record month for Instant Messenger worms.