With a large number of computer hosts already infected with Fizzer, especially among home users, the corporate networks managed by companies like MessageLabs may continue to be bombarded with e-mail messages generated by Fizzer, even as the number of new virus submissions from corporate and home users affected by the virus declines, according to Sunner.
He likened the problem to the use of compromised machines by spammers to send out junk e-mail messages. Sixty percent of those messages come from so-called "open proxies," machines that have been compromised by viruses and left open to use by spammers, Sunner said.
"(Fizzer) will be kept alive by the home user market because they're the slowest to update (antivirus software) and clean the virus up," he said.
Fizzer also infects the shared files folder used by the Kazaa peer-to-peer file sharing application and is capable of spreading over the Kazaa network and through vulnerable shared directories on computer networks, according to AVERT Labs at Network Associates.
While that made the virus something of an anomaly, the vast majority of new infections came from individuals clicking on e-mail attachments rather than the Kazaa network. That fact, alone, may account for Fizzer's rapid decline, according to Belthoff.
"This is not any sort of aggressive kind of Slammer worm. This is something that requires human interaction and an unprotected system to spread," he said.