The Sobig e-mail worm, which struck in August, was the most prolific virus of 2003, according to a top 10 list of viruses published by antivirus software company Sophos.
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The the Sobig-F variant accounted for almost 20% of virus reports to Sophos in 2003, easily beating Blaster, which notched up 15%.
A Blaster derivative, Nachi, also known as Welchia, took third place, followed by Gibe at number four and Dumaru in the fifth spot.
Sophos ranks viruses by the number of infection reports it receives from customers, which may have tipped the scales in Sobig's favour. Unlike Blaster, Sobig used e-mail to spread and generated massive volumes of e-mail traffic once it infected machines.
All those e-mail messages caught the attention of companies, some of which reported receiving hundreds of thousands of infected messages a day.
"Blaster was an internet worm, so if people applied the necessary Microsoft patches, they didn't get infected," said Carole Theriault, a security analyst at Sophos.
"With Sobig, the flow of e-mail was there regardless of whether you had the proper software patches and antivirus updates."
Sophos itself received more than 400,000 Sobig e-mail messages within the first 24 hours after the worm appeared in the wild, she added.
The company was also flooded by calls from customers in the days after the worm began to spread, which vaulted Sobig-F, the sixth version of the worm to appear on the internet, to the top spot.
However, translating the number of phone calls into information on the number of systems infected with Sobig is difficult, if not impossible, she said.
"It's difficult to say 'this many people were infected', and that can be misleading. There are a lot of people infected [with Sobig] now who don't know it and are continuing to spread the virus,"
Other prominent virus outbreaks did not make it to the list, including the Slammer worm, which appeared in January and targeted Microsoft's SQL Server.
While that worm ranked highly, reports of infections dropped off quickly after the worm initially appeared, as organisations disinfected and patched vulnerable SQL Server installations.
All of the top 10 viruses targeted Microsoft's Windows operating system, a trend likely to continue in 2004, according to Sophos.
Spam will also continue to plague e-mail users next year, with spammers adopting new techniques to get their messages past antispam products and filters, Sophos said.
Paul Roberts writes for IDG News Service