The Sober-I worm has had a big impact on November's chart of most dangerous viruses and worms, according to security firm Sophos.

Although the worm was only detected on 19 November, it rapidly rose through the ranks to account for 20.4 per cent of all infections reported to Sophos. This made Sober-I the second most common virus after Netsky P.

More than one in five of all emails scanned contained some form of virus last month, Sophos reported.

"Despite the appearance of Sober-I on the scene, relative old timers Netsky-P and Zafi-B have continued spreading, even though protection against them was made available as soon as they were detected," said Carole Theriault, security consultant at Sophos.

"Users can thwart these established threats simply by keeping their antivirus protection up to date."

The Netsky P worm, which harvests email addresses and spreads via peer-to-peer traffic, was first discovered in March and effective protection has been available from all antivirus firms since then. It accounted for almost a quarter of all detected worms and viruses last month.

Meanwhile the Hotmail hoax is still proving popular, accounting for nearly a quarter of all email hoaxes detected and now in its fifth month at number one in the company's hoax chart.

The scam involves an email purporting to be from Hotmail administration checking to see whether accounts are active and urging recipients to pass the mail on to other Hotmail users.

The top 10 viruses and hoaxes reported to Sophos in November 2004 can be seen here. More information about Sober-I can be found here