The Sasser worm was the most prevalent malware in May, a month which was inundated by new viruses, according to antivirus vendor Sophos.
Sasser accounted for more than half of the malware reports received by Sophos last month. Variations of Netsky accounted for most of the other half.
In May, Sophos said it analyzed 959 new viruses. That is the highest number of new viruses discovered in a single month since December 2001, the company said.