The company, which screens corporate e-mail accounts for viruses, said it intercepted more than 2 million infected messages in the first six months of 2002, double what it encountered in the same period last year.
Various forms of theworm were the worst offenders, responsible for more than half of all infected messages. Klez, which started off as a fairly innocuous mass-mailing worm, mutated into nastier versions as virus writers tried to improve on the original code, resulting in the pandemic variant.
John Harrington, U.S. marketing director for MessageLabs, said such serial development of worms has helped virus writers create more malicious pests.
"These guys are all learning from each other," Harrington said. "They find something that works; they add their own little social engineering twist, and the worm keeps going.
"Not only are we seeing more e-mail viruses out there, but the type we're seeing are more malicious," Harrington added. "Two years ago, the big pest was the; it was a mass mailer, but the payload wasn't that malicious. With SirCam and Klez, they're extremely malicious--they root around your hard drive; they send out sensitive documents."
, which surfaced last year, was the second most active pest, responsible for 600,000 infections. Harrington said the worm's durability is partly the result of its concentration among home PC users, who often lack adequate security measures, and of the design of the worm.
"There's a lot of people out there who don't even know they have it," Harrington said. "These kind of viruses can just hang around in the background."
The report noted that although the United States and Europe continue to be the main source of virus outbreaks, the number of pests originating from Asian countries--particularly China and Taiwan--has grown substantially.
"More countries, including the U.S., are starting to crack down on virus writers," Harrington said, "so more of them are originating in countries that don't have very strict laws on these things."