Security firm Panda says 25 percent of all new worms are designed to spread through portable storage devices such as USB drives.

"Much of the malware in circulation has been designed to distribute through these devices," said Luis Corrons, the technical director of PandaLabs, the research arm of Panda Security. "Not only does it copy itself to these gadgets, but it also runs automatically when a USB device is connected to a computer, infecting the system practically transparently to the user."

While a quarter of all 2010's worms rely on USB devices to spread to other PCs, a recent Panda survey of more than 10,000 small- and medium-sized firms found that 27 percent of those hit by a malware infection in the last year reported that the attack had originated with infected USB hardware, primarily flash drives.

Other devices that connect to PCs via USB, including smartphones, cameras and music players, also are a threat, added Corrons. "All these devices have memory cards or internal memories and therefore it is very easy for your cell phone, say, to be carrying a virus without your knowledge," he said.

The Stuxnet worm was one of the year's high-profile threats that relied on USB drives. In July, Stuxnet targeted PCs running software that managed large-scale industrial control systems in major manufacturing and utility companies by exploiting a then-unpatched vulnerability in Windows's shortcut files.

When users viewed the contents of an infected USB drive with a file manager like Windows Explorer, Stuxnet loaded itself onto the PC.

Microsoft issued an emergency "out-of-band" security update on Aug. 2 to plug the shortcut hole.

The USB infection vector isn't new. Two years ago, the Conficker worm made headlines worldwide after it spread using flash drives, among other avenues.