Top 10 viruses: A Christmas special

The virulent Badtrans worm has trounced the other viruses to take the number one spot in December's top 10, the busiest month ever for virus activity.

The worm first hit users back in November and has eclipsed other more recent outbreaks, such as the Goner virus, to gain first place in the league of shame. According to figures provided by anti-virus firm Kaspersky Labs, Badtrans accounted for almost nine out of 10 reported viruses in December.

Alex Shipp, anti-virus technologist for MessageLabs, said the worm had been so successful because of clever social engineering which made it difficult to spot.

He said: "People really need to make sure their anti-virus software is up-to-date to combat this one - it's very hard to battle against otherwise."

The worm spreads by replying to emails in a victim's inbox, making it look as if the message is solicited, and sending an attachment, which can have any one of 30 names.

In addition it fakes the sender's address, meaning people receiving the virus can't let the person they've caught it from know they have it.

This means victims won't necessarily know they've been hit for some time.

Badtrans easily beat the Goner virus to the top spot for the month of December, despite Goner initially being one of the quickest growing viruses in history. Goner fizzled out after three days as users got used to the format.

Shipp added: "The relative performance of Goner and Badtrans shows how important social engineering is in getting a worm to spread. Goner is easier to pick up on so it died."

Click here for the full top 10: http://www.silicon.com/a50192 .