For years now, Alex Haddox, of the Symantec Antivirus Research Center, in Santa Monica, Calif., has studied the devastating effect of boot-sector viruses. Those viruses infect the files that allow the computer to operate, and they can be transferred only by human contact, that is, by floppy disk. But boot-sector viruses don't keep Haddox up at night. There's a new kind of virus that's even worse.

The latest threat, dubbed a macro virus, burrows into the code of such applications as word processors. Unlike its predecessors, it can easily migrate across most platforms. Their devilish lines of code, simple to write, can sit dormant on a computer for years before they bring down an entire network. Worst of all, a macro virus could spread across the Internet at a terrifying pace.

The first macro virus to be identified was called the "concept virus." It's the most prolific virus in history. That particular virus does no harm beyond printing a secret message, "And that is enough to prove my point," in the computer's code.

Macro viruses get mainlined into computers around the world every day. A bandit infected a computer at the University of Auckland, in New Zealand. The Antivirus Research Center was summoned, but in only two weeks the virus had spread to six countries. "We cured it, but not before it had reached as far as South Africa, England, and the United States," says Haddox. It's a good argument for using and frequently updating your company's virus-protection software.