Computer viruses are mysterious and grab our attention. Every time a new virus hits, it makes the news if it spreads quickly. On the one hand, viruses show us how unknowingly vulnerable we are. A properly engineered virus can have an amazing effect on the world-wide Internet. On the other hand, they show how sophisticated and interconnected human beings have become. For example, the "Melissa" virus -- which became a worldwide phenomenon in March of 1999 -- was so powerful that it forced Microsoft and a number of other very large companies to completely turn off their e-mail systems until the virus could be contained. The "ILOVEYOU" virus in 2000 had a similarly devastating effect. That's pretty impressive when you consider how simple the Melissa and ILOVEYOU viruses are! When you listen to the news, you hear about many different forms of electronic infection. The most common are: Viruses - A virus is a small piece of software that piggy-backs on real programs. For example, a virus might attach itself to a 
program like a spreadsheet program. Each time the spreadsheet program runs, the virus runs too, and it has the chance to reproduce (by attaching to other programs) or wreak havoc. Email viruses - An email virus moves around in email messages, and usually replicates itself by automatically mailing itself to dozens of people in the victim's email address book. Worms - A worm is a small piece of software that uses computer networks and security holes to replicate itself. A copy of the worm scans the network for another machine that has a specific security hole. It copies itself to the new machine using the security hole, and then starts replicating from there as well. Trojan Horses - A trojan horse is simply a normal computer program. The program claims to do one thing (e.g. - it claims to be a game) but instead does damage when you run it (e.g. - it erases your hard disk). Trojan horses have no way to replicate automatically.