How has the influence of the online world (the Internet, networks, bulletin board systems, and e-mail) affected the world of viruses?

Since November 1987 when the Internet Worm created by Robert T. Morris, Jr. wreaked over $100 million worth of problems on computers connected to the Internet, it has been obvious that having computers connected on a computer network increased the possibility of virus infections. That is not to say that networks are bad, in fact networks are becoming more and more essential and valuable all the time. It's just that being online on a network, a BBS, or dialed into a remote computer increases the possibility of contact with viruses.