November was the heaviest spamming month of 2005, a leading message security firm said this month.
CipherTrust attributed the peak to the latest outbreak of the Sober worm, which the company said was responsible for more than 500,000 PC infections.
The firm cited a number of specific outbreaks as highpoints for spam volume during the month.
The Mytob and Bagle outbreaks on Nov. 3 resulted in an 8-percent spike in the amount of spam from the night before. Eighteen days later, the latest Sober outbreak infected thousands of new zombie PCs, increasing the amount of spam traffic by 15 percent.
Spam was also on the menu over the Thanksgiving holiday weekend. Spam email volumes were up 25 percent over that of the previous weekend.
The number of new zombie PCs sending spam and viruses increased by nearly 50 percent, with an average of 250,000 new infected accounts daily.
Paul Judge, CipherTrust chief technology officer, said November bucked a trend of declining spam numbers.
"The propagation of these threats proves spammers and attackers continue to be as malicious as ever. This is particularly true as the exponential increase in unwanted email costs money in wasted bandwidth capacity, storage and dangerous payloads, as well as valuable administration and employee time," he said.
Postini has named Sober the most widespread virus ever, saying that it quarantined 218 million Sober-infected messages over a week-long period last month. The company also said it intercepted 29 million copies of the virus in a 24-hour span.