If only that were enough to keep the Internet free from the wanton devastation of Code Red II and Nimda, just two of the new automated menaces (both technically worms, rather than viruses) now infecting millions of computer networks. Security experts admit such attacks can't be prevented entirely, but they say simulation technologies now in development might at least help network operators predict how their systems will respond to invaders, so they can prepare better defenses and contain the damage.

The latest rashes of corrupting code are particularly virulent because they don't require any social engineering-a phrase used to describe how virus makers trick people into opening tainted e-mails-and can infect networks without anybody noticing. Code Red II scans the Internet for vulnerable Web servers and creates "back doors" that allow hackers to control the servers remotely, to date causing $2 billion worth of server downtime and Internet traffic jams. Nimda spreads automatically via shared files, Web pages, e-mail and other routes. Infected computers can be cleaned, but the worms spread with such speed and in such volume that networks can grind to a halt.