The Internet may be 40 years old, but one of the most infamous aspects of both the Web and personal computing in general is quite a bit younger. It was 26 years ago when the first known proof of concept code for a computer virus was demonstrated in a public venue, following up on theories and other attempts to produce self-replicating code. The first official "virus" was born then, giving way to a fundamental change in computer security.
A few years after the introduction of the first virus came the first widely-propagating worm, which spread itself over thousands of machines across the world (which represented a substantial portion of Internet-connected machines at the time). It wasn't until many years later, when an Internet connection was more common in households that the idea of mass-scale computer infections popped into people's heads. Now, we hear dozens of stories a year about some new threat, which have goals ranging from draining your bank account to taking command of millions of PCs for denial of service attacks.
The evolution of the computer virus has always been interesting to watch, even if it's indication that no matter how hard security companies try, there is always going to be new, smarter threats to deal with.