But the actual number of infections could be higher or lower than 4.6 million, DiMino admitted. Because the Working Group's method counts IP addresses, they may have over-counted consumers who log on from multiple IP addresses, or undercounted corporate infections, which are often hidden behind a single IP address.

OpenDNS, IBM and the Working Group all used different techniques to arrive at their estimates, but they all rely on the fact that infected machines need to check in with a "command and control" server for instructions.The Working Group got its data by setting up "sinkhole" servers at points on the Internet used by infected machines to download instructions. They did this by taking over the Internet domains that Conficker is programmed to visit to search for those instructions.

The number of infections measured by the Working Group is in line with its estimates of earlier variants of the worm, DiMino said. "Not all of the As and Bs have been turned into Cs," he said.