For years computer scientists have dreamed of building computer security software as effective and versatile as the human immune system, which can swiftly overwhelm even intruders it has never seen before. Such a software tool would free programmers from having to anticipate and respond to the changing array of attacks hackers use; instead, it would simply allow a computer to understand its own normal behavior and shut down deviant processes.

Now such systems are finally going commercial. An intrusion prevention system from Sana Security of San Mateo, CA, is protecting private customer data for companies like Smith and Hawken, a catalogue and Internet retailer in Marin County, CA. Smith and Hawken officials say none of the many attempts to infiltrate their computers have succeeded since they began a trial run of Sana's software in mid-2002, and that hands-on monitoring has been reduced by dozens of hours a week. "I've heard the immune system analogy before in the security business, and usually my eyes roll up in my head," says Eric Ogren, a senior analyst and computer security specialist at Boston technology consultancy the Yankee Group. "What makes Sana unique is the way their software is able to automatically learn."