Computer experts, fending off the largest assault ever on America's computers, scrambled to patch up a potential nationwide security problem exposed by a "virus" that quickly spread through hundreds of university and government research computers.

No permanent damage or security breaches appeared to have occurred during the attack, but many computers were slowed by loads of useless viral information, then had to be shut down Thursday so they could be "disinfected."While the exact identity of the saboteurs remained a mystery, an anonymous caller to the New York Times said the virus apparently was the result of an experiment by a computer science graduate student who slipped the virus into the Advanced Research Projects Agency Network computer system.

As the virus spread to other computer systems, experts said it was the worst attack of its kind in the nation's history.

"This is going to be the most extensive virus that has ever invaded any computer system," Greg Chartrand, manager of the computer network at Fermi National Accelerator Laboratory, told the Washington Post.

The invader, termed a "virus" or "worm" was a small set of computer instructions, probably created by a computer hacker to expose the potential security problem, experts said.

Such invaders are designed to slip unwanted instructions into computers invisibly, making the invasion undetectable until it causes problems. This "virus" forced the computers to make many copies of the viral program and send the copies, which could also "reproduce," to the other accessible computers, packing them with the useless viral information.

Computer experts at Lawrence Livermore National Laboratory - one of the facilities invaded by the bug - deemed the invader a "worm" because it did not appear to be destroying information stored in the computers, as a "virus" would. Other experts simply called the invader a virus.