YOKOTA AIR BASE, Japan — U.S. military officials were mum Thursday about a computer virus that attacked e-mail systems globally earlier this week.

The “F” variant of “Sobig” virus has been labeled the fastest-spreading e-mail virus in history. MessageLabs Inc., which scans e-mail for viruses, said that within 24 hours it had scanned more than 1 million copies of the virus, which was blamed for computer disruptions at businesses, colleges and other institutions worldwide.

There have been faster outbreaks on the Internet, but those circulated through networking functions built into Windows operating systems.

The Sobig virus spreads when computer users open file attachments in e-mails with headings such as “Thank you,” “Re: Details” or “Re: approved.” Once the file is opened, Sobig scours the computer for e-mail addresses. It then sends scores of messages to the addresses it has collected.

Sobig does not physically damage computers, files or critical data, but it can clog e-mail services with messages, forcing users to spend inordinate amounts of time deleting them. It also deposits a Trojan horse, or hacker back door, that can be used to turn computers into spam magnets.

Sobig began appearing Tuesday, just a week after a separate virus, “Blaster,” wreaked havoc on computer systems across the world.

Citing security concerns and policy, U.S. Forces Japan officials declined to comment Thursday on the worldwide outbreak or whether any of their systems had been affected. U.S. Forces Korea officials had no comment as of press time.

But Yoichi Tomita, the Army and Air Force Exchange Service’s computer repair shop owner, said viruses do not discriminate. “A computer is a computer.”

Tomita estimates his shop received about 50 inquiries in the past week concerning the viruses “msblast.exe,” nicknamed the “lovesan” bug, and “msblast-d,” often called the “welchia” virus. In five instances, customers had to bring in their hard drive’s CP unit for repair.

“There’s many nicknames for viruses, but they follow the same security hole,” Tomita said. “They’re much worse-spread and much faster than before.”

— The Associated Press contributed to this report.

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