November was the worst month of the year for spam email, according to a survey from messaging security firm CipherTrust.

Led by the Sober virus, hundreds of thousands of computers have been turned into zombies, spewing floods of unwanted email, CipherTrust said.

On one day, Nov. 3, outbreaks by Mytob and Bagle triggered an 8 percent surge in email around the world, CipherTrhust added.

On Nov. 21, the latest outbreak of the Sober virus triggered a 15 percent increase in spam, the company said. Over Thanksgiving weekend, email volume surged 25 percent more than the weekend before.

Some 170,000 newly infected computers are being turned into zombies each day as a result og the Sober attack, CipherTrust added.