The program, known as Conficker, uses flaws in Windows software to co-opt machines and link them into a virtual computer that can be commanded remotely by its authors. With more than five million of these zombies now under its control — government, business and home computers in more than 200 countries — this shadowy computer has power that dwarfs that of the world’s largest data centers.

Alarmed by the program’s quick spread after its debut in November, computer security experts from industry, academia and government joined forces in a highly unusual collaboration. They decoded the program and developed antivirus software that erased it from millions of the computers. But Conficker’s persistence and sophistication has squelched the belief of many experts that such global computer infections are a thing of the past.

“It’s using the best current practices and state of the art to communicate and to protect itself,” Rodney Joffe, director of the Conficker Working Group, said of the malicious program. “We have not found the trick to take control back from the malware in any way.”…

There is also a different possibility that concerns the researchers: That the program was not designed by a criminal gang, but instead by an intelligence agency or the military of some country to monitor or disable an enemy’s computers.