A new digital plague has hit the Internet, infecting millions of personal and business computers in what seems to be the first step of a multistage attack. The world’s leading computer security experts do not yet know who programmed the infection, or what the next stage will be.

In recent weeks a worm, a malicious software program, has swept through corporate, educational and public computer networks around the world. Known as Conficker or Downadup, it is spread by a recently discovered Microsoft Windows vulnerability, by guessing network passwords and by hand-carried consumer gadgets like USB keys.

Experts say it is the worst infection since the Slammer worm exploded through the Internet in January 2003, and it may have infected as many as nine million personal computers around the world.

Worms like Conficker not only ricochet around the Internet at lightning speed, they harness infected computers into unified systems called botnets, which can then accept programming instructions from their clandestine masters. “If you’re looking for a digital Pearl Harbor, we now have the Japanese ships steaming toward us on the horizon,” said Rick Wesson, chief executive of Support Intelligence, a computer security consulting firm based in San Francisco.