Undated (AP) _ Universities and research centers turned computers back on Friday after sweeping out a ''virus'' epidemic that infected machines across the country, a ''royal nuisance'' that slowed research but apparently caused no damage.

''Everything seems calm this morning,'' said Bob Logan, systems manager at the California Institute of Technology in Pasadena. ''It's just a lot of aggravation right now. ... We want to look and make sure that besides doing what it (the virus) did it didn't leave some little time bomb that's going to go off.''

The virus, a sort of rogue program that reproduced in computers and apparently instructed them to send copies of itself to other machines through a research network, ARPANET, slowed computers and used up memory space. But most operators said the biggest problem was the time wasted while eliminating it.

About 1,000 hours of staff time was spent cleaning up some 200 computers at the University of Colorado, said public relations director Pauline Coker.

''There was no real damage. The program is not the big deal - the big deal is that it happened,'' said Hans Werner Braun, chief programmer for the University of Michgan.

The New York Times reported Friday that an anonymous caller to the newspaper claimed that a graduate student had created the virus as a harmless experiment and became terrified of the consequences when it got out of hand.

Once Caltech's computers were disconnected from ARPANET's electronic mail system and turned back on Thursday night, ''we could watch the virus trying to spread from infected machines on the network,'' Logan said.

ARPANET, established by the Defense Department's Advanced Research Projects Agency, is used by researchers and defense contractors to share data at high speed.

The virus also spread into some government computers, from NASA's Langley Research Center at Hampton, Va., to the Los Alamos National Laboratory in New Mexico. Computers used for classified work are separate from those affected by the virus.

Computers at the Lawrence Livermore National Laboratory, one of the nation's nuclear weapons design centers, were shut down late Wednesday, but turned back on and reconnected to ARPANET Friday morning, said spokeswoman Bonnie Jean Barringer. No data was lost but research was disrupted for the day, she said.

Purdue University computers also were restarted Friday, but slowly. ''We hope there are no Trojan horses hidden,'' said Steve Hare, research facilities manager of Purdue's computer science department.

Boston University computers were back on Friday but had not been linked back up with the network, said Mary Riendeau, coordinator of user services for the computer science department.

Many institutions didn't discover the virus until Thursday, but Los Alamos scientists detected it Wednesday evening and isolated their computers from the network.

''It was noticeable, but not catastrophic,'' said Los Alamos computer security officer Jimmy McClary. He said there was no damage but a few hundred scientists suffered delays in their work.

''We were warned yesterday morning to board up the windows and protect ourselves,'' said Leslie Maltz, director of computing and communication resources at Stevens Institute of Technology in Hoboken, N.J. He would not say who gave the warning.

''We'll keep systems removed till all clear is sounded, several days at most I would think,'' Maltz said.

Carnegie Mellon University computers in Pittsburgh were innoculated five years ago with programs that recognized the virus and tried to send the messages back to their return address. When that proved to be an improper format the system channeled the virus messages to a file to be examined by humans, said Bob Cosgrove, director of computing services at CMU.

But at the University of Illinois, the virus was still causing trouble Friday.

''We went home last night thinking it wouldn't come back because we had closed the hole it was coming in through, but it apparently has multiple ways of attacking the system and we got hit again,'' said Charley Kline, senior research programmer at the university.

It appeared that most infected machines were either Digital Equipment Corp. VAX or Sun Microsystems computers, and all used BSD UNIX operating system, the master program that lets a computer run other programs, said Logan at Caltech.

It was ''one of the most complex viruses we've seen,'' said John McAfee, director of the Computer Virus Industry Association in Santa Clara, a group of 11 computer companies that market anti-viral programs.

''It's been a royal nuisance ... but of the spectrum of things that could have happened it was the least destructive thing that could have been done,'' said Bill Johnston, a computer scientist at the Lawrence Berkeley Laboratory, part of the University of California-Berkeley.

McAfee said the virus took advantage of a little-known flaw in the send- mail feature of the UNIX operating system as modified by UC-Berkeley. ''The perpetrator was an extremely sophisticated individual,'' he said.