July 2004 has the dubious honour of being the month that saw the largest number of new viruses recorded in the wild since December 2001, security experts have observed.

According to security firm Sophos a total of 1,157 previously undocumented viruses were recorded last month - the highest figure for three years.

The company said that the Hungarian Zafi worm topped the list of most prevalent infections during July, and is still spreading at "staggering rate".

Zafi was found to have accounted for more than half of all viruses spotted by Sophos monitoring stations during the month.

"The Zafi worm, which can arrive in several different languages, won't be disappearing anytime soon," said Carole Theriault, security consultant at Sophos.

"There are so many copies of Zafi.b flying around, trying to break through companies' defences. It is vital that computer users around the world are protected fully to ensure worms like these don't hang around like a bad smell."

MyDoom.o, first seen on 26 July, took only a few days to burst into the virus chart top 10. Its rapid spread has been attributed to its unprecedented technique of infecting computers: scooping up email addresses and polling search engines for more addresses to bombard.

"MyDoom.o hit Google particularly hard, and millions of users were unable to access the search engine," added Theriault.

It is not simply the record-breaking number of new viruses that is posing a threat, but rather the fact that traditional viruses are becoming more dangerous by adding spam to their payloads, according to security firm MessageLabs.

"The virus and spam landscapes have changed dramatically. Virus writers and spammers are combining their skill sets to produce a more sophisticated breed of email security threat, one in which the lines between viruses and spam have become increasingly blurred. This fusion of email attack methods is already widespread," MessageLabs warned.

The firm pointed to viruses designed to aid the spread of spam, such as Fizzer, Bugbear, the SoBig worms and MyDoom, as illustrating this trend towards blended spam and virus threats.

According to MessageLabs almost every virus it has intercepted during 2004 has lent itself to potential spam distribution.

The Virus Top 20 for July, compiled by antivirus experts Kaspersky Labs, can be viewed here.