The Sasser worm has come first in anti-virus firm Sophos’s May top-ten virus chart.

Several versions of the Netsky worm took places from six places in the chart.

"Sasser proved to be a major nuisance in May, affecting even more users than even the Netsky worms," said Chris Kraft, senior security analyst at Sophos.  "The best way to stop such worms is to deploy a consolidated defence. This includes multi-level anti-virus protection, firewalls and policies to update operating systems."

The report, which examined user virus reports, showed that the Sasser worm accounted for more than half the reports received by Sophos.

The top ten viruses in May 2004 were as follows:

1.  W32/Sasser                     51.1%  NEW ENTRY
2.  W32/Netsky-P                 11.7%
3.  W32/Netsky-B                  4.1%
4.  W32/Netsky-D                  3.8%
5.  W32/Netsky-Z                  3.4%
6.  W32/Netsky-Q                  2.4%
7.  W32/Netsky-C                  2.1%
8.  W32/Sober-G                   1.5%  NEW ENTRY
9.  W32/Bagle-AA                  0.8%  NEW ENTRY
10. W32/Lovgate-V                0.7%  NEW ENTRY

The firm discovered 959 new viruses in May, which is the highest number in a single month since December 2001.