One report identified the virus as W32/SillyFCD-W, which spreads via thumb drives that move from machine to machine. Once the virus is on a machine, it may download other malware, according to Sophos. Protection against the worm has been available from Sophos since April 2007.

Perhaps in a related move, the department has issued a directive banning thumb drives from its networks, leading to speculation that the virus may have been traced to one, reports say. Thumb drives have been a source of .

While a military spokesman has confirmed an outbreak, he would not discuss details as per defense policy, according to published reports.

Banning thumb drives is a relatively routine policy for organizations interested in security, reliability and data loss prevention, one analyst says. "That's not necessarily an out of the ordinary policy, says Mike Montecillo, a security analyst for Enterprise Management Associates.

One concern is that employees can attach at thumb drive and download sensitive data and then walk off with it, he says. Or the thumb drive could be used on any number of machines, exposing it to infection that it can then spread. "Who knows what crept onto the thumb drive while it was attached?" he says.