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The Digital Common Cold

Much mystique surrounds viruses and virus writing. But viruses are no real mystery. Nearly anyone with a bit of programming knowledge can write one in less time than it takes to download a Moby track from Napster.

Simply put, a virus is a piece of code that instructs your computer to do something (called the "payload") that you don't want it to do. The payload can be as harmless as inserting an inane quip from South Park into your department's status report. Or it can be as damaging as erasing your hard disk or installing a secondary program that lets someone else take over your system.

One defining characteristic of a virus is that it replicates itself. It contaminates your system by copying itself into programs, documents, or system files, in the same way that a biological virus copies itself and attacks organs in your body. Once a virus infects these files, its payload can be triggered to go off at any time or in response to an activity such as opening Word or hitting Ctrl-P. One part of the Melissa payload activated whenever the time of day matched the date. So at 4:10 on April 10, a quote from The Simpsons would pop up on the screen.

Macro viruses--those written in macro programming language--account for some 75 percent of viruses in the wild because they're easy to write and disseminate. (For more about types of viruses see "How It Works: Viruses.")

Two other types of malicious code--worms and Trojan horses--share some characteristics with viruses but aren't categorized as such. A worm replicates like a virus but doesn't alter files or zap data. Instead, it slithers through network connections, collecting addresses of other systems on the network and passing copies of itself from PC to PC, thereby clogging the network.

Unlike viruses and worms, a Trojan horse doesn't replicate. It's a malicious program that comes disguised as something benign, such as a screen saver or a chess game. When loaded onto your machine, it can capture information from your system or allow someone else to commandeer your computer remotely.