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Computer viruses have been a fact of life at least since the 1980s, if not before. They can cause companies to lose hours of working time and they can also spread panic among computer users everywhere. There are, however, several distinct types of computer infection - all loosely referred to as viruses - and they each work in a slightly different way. A particularly nasty one is the worm, which is a program designed to sneak its way into an entire computer network, and reproduce itself over and over again. Then there is the Trojan, which strictly speaking isn't a virus, but a piece of software that appears to do one thing, but actually does something malicious instead. When the [unsuspecting] operator introduces it into the computer, the alien program will take over the machine. With Trojans you have to be particularly careful because they can often be introduced by way of a message advertising an anti-virus product. So what motivates someone to introduce a virus into the computer systems of innocent victims? Perhaps it's simply the desire to prove that it can be done. Or because it gives the kind of pleasure you get from solving a difficult problem - nowadays people protect their computers with all sorts of security software, so it takes considerable skill to break through all the defences and introduce a virus.
