How Computer Viruses Work

Computer viruses are mysterious and grab our attention. On the one hand, viruses show us how vulnerable we are; a properly engineered virus can have an amazing effect on the worldwide Internet. On the other hand, they show how sophisticated and interconnected human beings have become.

For example, the things making big news at the end of 2003 were the MSBlaster worm and the SoBig virus. The Melissa virus - which became a global phenomenon in March 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 that the Melissa and ILOVEYOU viruses are incredibly simple.

This document looks at both "traditional" viruses and the newer e-mail viruses in order to understand how they work and also understand how to protect against them. Viruses in general are on the wane, but occasionally a person finds a new way to create one, and that's when they make the news.

Types of Infection

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 piggybacks on real programs. For example, a virus might attach itself to a program such as 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.
• E-mail viruses - an e-mail virus moves around in e-mail messages, and usually replicates itself by automatically mailing itself to dozens of people in the victim's e-mail 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 computer program. The program claims to do one thing (it may claim to be a game) but instead does damage when you run it (it may erase your hard disk). Trojan horses have no way of replicating automatically.

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