After scanning 2 million computers over the past 24 hours, IBM's Internet Security Systems (ISS) division that it had spotted the worm on 4 percent of the IP addresses it monitored.

Although Conficker is clearly the worst worm outbreak in years, the results came as a surprise, according to Holly Stewart, a threat response manager with ISS. "It is higher than what we expected; I thought we'd see 1 to 2 percent," Stewart said.

Late last week, IBM researchers reverse-engineered Conficker and figured out a way to track infections by measuring peer-to-peer traffic on the network. They used that technique to reach their estimate.

The results are similar to numbers released Wednesday by OpenDNS, which said it had also spotted a much larger number of infections than expected. Both IBM and OpenDNS' numbers count Conficker.C, the latest variant of the worm, and one that is easier to spot communicating on the network.

Conficker began spreading in October 2007, using a handful of sneaky tricks to spread. Once it infects a machine, it can spread very quickly on a local area network by taking advantage of a now-patched flaw in Microsoft Windows.