One in every 13 email messages is infected with the latest variant of the Sober worm, according to a leading online security company. Postini has quarantined 218 million Sober-infected messages over a seven-day period last month, and has said that it has intercepted 29 million copies of the virus in a 24-hour span.

"We typically quarantine about 50 million virus-infected emails in a month. This Sober virus generated close to a 1,500-percent increase in virus-infected email traffic in the past week," said Scott Petry, Postini founder and vice president of products and engineering.