Security experts say the malicious software is apparently being loaded at the final stage of production, when gadgets are pulled from the assembly line and plugged in to a computer to make sure everything works.

If the testing computer is infected — say, by a worker who used it to charge his own infected iPod — the digital germ can spread to anything else that gets plugged in.

The recent infections may be accidental, but security experts say they point out an avenue of attack that could be exploited by hackers.

"We'll probably see a steady increase over time," said Zulfikar Ramzan, a computer security researcher at Symantec Corp. "The hackers are still in a bit of a testing period — they're trying to figure out if it's really worth it."

Thousands of people whose antivirus software isn't up to date may have been infected by new products without even knowing it, experts warn. And even protective software may not be enough.

In one case, digital frames sold at Sam's Club contained a previously unknown bug that not only steals online gaming passwords but disables antivirus software, according to security researchers at CA Inc.

"It's like if you pick up a gun you've never seen before — before you pull the trigger, you'd probably check the chamber," said Joe Telafici, vice president of operations of McAfee Avert Labs, the research arm of security software maker McAfee Inc.

"It's an extreme analogy, but it's the right idea. It's best to spend the extra 30 seconds to be sure than be wrong," he added.

Consumers can protect themselves from most factory-loaded infections by running an antivirus program and keeping it up to date. The software checks for known viruses and suspicious behaviors that indicate an attack by malicious code — whether from a download or an attached gadget.

Monitoring suppliers in China and elsewhere is expensive, and cuts into the savings of outsourcing. But it's what companies must do to prevent poisoning on the assembly line, said Yossi Sheffi, a professor at the Massachusetts Institute of Technology.

"It's exactly the same thing, whether it happened in cyberspace or software or lead paint or toothpaste or dog food — they're all quality control issues," Sheffi said.

While manufacturing breakdowns don't happen often, they have become frequent enough — especially amid intense competition among Chinese suppliers — to warrant more scrutiny by companies that rely on them, Sheffi said.