The worm has kept Josef Chamberlin busy at the keyboard, operating on only snippets of sleep, many recent days and nights.

But really, if it weren't this worm, it might be another computer worm, a virus, or simply an idea that would push Chamberlin out of bed toward the glow of his monitor. Chamberlin works for EDS in Rancho Cordova, an international electronic data management company. His specialty is security and intrusion detection -- a trade he learned in youth as a hacker. His skills are in high demand and becoming more so. The worm dubbed "Code Red" is the newest example of why. Twice in two weeks, this bit of digital mischief has threatened to bring the Internet to its knees. The worm hijacks Web sites that use certain Microsoft server software, slowing traffic in its search for other servers to infect. Microsoft developed a modification to patch the "hole" through which the worm enters. This week, the software maker, along with the FBI and others, implored computer users and systems administrators to apply the patch before the worm reawakened Tuesday night. The warning seemed to work. Although an estimated 150,000 systems were infected, Internet traffic flowed mostly smoothly Wednesday. The danger, in Chamberlin's estimation, remains real. "It really has been the biggest threat to the Internet in modern times," he said. A worm is more clever than a virus in that it actively searches for flawed software and requires no action on the part of users, such as clicking on an attachment, to spread. But the bigger reason this worm is such a threat is that there are so many people conducting business online today. About 4 billion addresses reside on the Internet. All that business attracts hackers, whether criminals, mischief-makers, self-appointed agents for social change or just curious observers. Chamberlin knows this because he's one of them.Chamberlin has been a hacker since before there were computers. Out of high school, he joined the Air Force, then took a computer job with a furniture maker in Germany, followed by a stint in Japan working for AT&T. In 1998, he came home to California to take the job at the EDS Rancho Cordova site. In his interview with EDS, Chamberlin said, he showed them how he could hack into the system to their list of users and passwords. His point: The system needed improvement, and he could do the job.His boss, McKinney, said he has learned a few things about security from Chamberlin.