June 2004 has turned out to be one of the quietest months so far this year in terms of the number and severity of virus attacks, according to antivirus specialist Kaspersky Labs.

The company reported only one new entrant in its top 20 list of most prevalent viruses in June.

I-Worm.Zafi.b, which was written in Hungary, spread rapidly throughout Europe taking over from the Netsky family as the fastest spreading infection.

Kaspersky said that the most likely explanation for Zafi's success lies in its clever social engineering techniques.

The rest of the June top 20 is almost identical to May's, the security firm said, adding that the reason for this lull in activity is unclear.

"Maybe virus writers have been lying low due to the arrests of coders worldwide, or maybe antivirus vendors have succeeded in clearing up the aftermath of previous outbreaks," said Denis Zenkin, head of corporate communications at Kaspersky, in a statement.

Some email worms were found to have lost or gained a few points in the chart, but many remained in the same place.

"It is worth noting that Exploit.HTML.ObjData has gained strength, whereas Klez.h, a classic network worm, has finally disappeared from the list after a record breaking two-year stint," said Zenkin.

The company also warned that new threats come from viruses spreading via attacks on open ports rather than via email.

Kaspersky's Virus Top Twenty for June 2004 can be seen here.