Netsky-P took out the number one spot for viruses causing problems for businesses during July, according to a security firm.
The worldwide report, carried out by security firm Sophos, found that Mytob variants still accounted for seven of the top 10 viruses, spyware and spam reported to the company during July.
Taking out number two place in Sophos' top 10 for July was Mytob-AS, followed by Mytob-BE and Mytob-EP. Zafi-D came in fifth place.
Carole Theriault, security consultant at Sophos, said it wasn't the viciousness of the worm which was the problem, but the "constant onslaught of slightly differing variants".
"This month's only new entry is another family member -- Mytob-CX -- and despite accounting for only 3.2 percent of viruses in July, it shows that the Mytob team isn't about to go away."
"The Mytob army of malware is one of the biggest we have ever seen," Theriault said. "The criminals behind these attacks are constantly releasing new viral code to do their dirty work for them."
The total number of new viruses identified by Sophos during July was 1380, according to a statement from the company.