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AUSTIN, TEXAS -- Fear the worm. The computer worm, that is.

That was the theme of a talk Symantec's director of research, Stephen Trilling, gave to attendees at Dell's Direct Connect conference here Thursday.

People are aware of the dangers computer viruses pose, but computer users need to be aware of another growing threat called a worm, he said. The two are somewhat similar, but they differ in one important way: the way they spread.

Viruses spread from one file to many on a single computer, but they don't move to another computer on their own, he said. A user has to do something to make it happen--whether it's accidentally sharing a contaminated diskette or inadvertently passing or receiving an infected file via e-mail. A worm, on the other hand, lives to go forth and proliferate.

"A worm is a program specifically designed to spread itself from one computer to another on a network," Trilling said.

A virus spreads only as fast as people exchange information, but a worm spreads at the speed of the Internet, he said. A worm is less interested in infecting additional files on a single PC and is more interested in reaching more PCs.

People often confuse viruses and worms, he said. Why did Melissa, ExploreZip, and Love Letter spread so fast? They were worms.

The ExploreZip worm came out last year. It arrived as an e-mail, and when you clicked it, the worm would sit on your system and respond to each incoming e-mail. It also spread from one shared drive to the next without any user intervention, he said.

Melissa hit in March 1999. Once it arrived on a system it would send itself out to the first 50 people in the user's address book. Thanks to mailing lists with hundreds of names, it spread to more than 50 people each time it went out, he said.

A History of the Worm

The first worm was created for beneficial purposes at Xerox, Trilling said. In 1982 the company created a worm to perform a variety of repetitive tasks on computers such as cleaning up temporary files. It was a very useful program, until it went bad. The worm began to crash systems, and Xerox had to create one of the first antivirus programs to get rid of it.

In 1987, a new worm called Christmas.exe spread throughout IBM's e-mail network, he said. It moved like the Love Bug and displayed a primitive Christmas tree on its victims' monitors.

And finally, in 1988 a Cornell computer science graduate released an infamous worm called the Morris Internet worm. It used known Unix backdoors to break into some 6000 systems. These backdoors had patches, but people hadn't installed them, Trilling said.

These early worms stirred people up, but had limited worldwide impact. However, today's worms are able to reach considerably more users thanks to combination of factors.