On Thursday, researchers at the volunteer-run Shadowserver Foundation , all infected by the known variants of Conficker.

They have been able to keep track of Conficker infections by cracking the algorithm the worm uses to look for instructions on the Internet and placing their own "sinkhole" servers on the Internet domains it is programmed to visit. Conficker has several ways of receiving instructions, so the bad guys have still been able to control PCs, but the sinkhole servers give researchers a good idea how many machines are infected.

Although Conficker is probably the computer worm most known about, PCs continue to get infected by it, said Andre DiMino, co-founder of The Shadowserver Foundation. "The trend is definitely increasing and breaking 7 million is pretty much of a landmark event," he said.

Conficker first caught the attention of security experts in November 2008 and received widespread media attention in early 2009. It has proved remarkably resilient and adept at re-infecting systems even after being removed.

The worm is very common in, for instance, China and Brazil. Members of the Conficker Working Group, an industry coalition set up last year to deal with the worm, suspect that many of the infected PCs are running bootlegged copies of Microsoft Windows, and are therefore unable to download the patches or Microsoft's Malicious Software Removal Tool, which could remove the infection.