Another virus could be released any time," said Steve Trilling, research director with the Security Response Team at Symantec Corp. SYMC.O , a U.S.-based security company. "We can never be complacent when one threat seems to die down."
Mikko Hypponen, manager of anti-virus research at Finland-based F-Secure Corp, said one of the five prior versions of Sobig surfaced before the previous version expired. Sobig.E began circulating June 25, one week before Sobig.D was set to expire, he said.
The first version of Sobig arrived in January and had no expiration date. It was followed about four months later by Sobig.B. More sophisticated versions followed one week to three weeks after each preceding version, according to Hypponen.
The latest version, Sobig.F, first emerged a week ago and spread to hundreds of thousands of Windows-based computers, Hypponen said. Some 200 million e-mails have been sent over the Internet by infected computers, he estimated.