An anti-virus company reckons the number of viruses being sent by e-mail is continuing to fall -- but according to security experts, better targeting of attacks means the overall threat of infection has increased.

MessageLabs found in its latest trends report that the ratio of e-mails containing viruses fell to one in 52 during September. The ratio has continued to fall since June when it stood at one virus in every 28 e-mails. The ratio was one virus in every 32 e-mails during July and one in every 43 e-mails during August.

However James Scollay, vice president of MessageLabs Asia Pacific, told ZDNet Australia that there may be fewer viruses, but they are more sinister in nature.

"Whilst the number of viruses is falling the purpose of those viruses is more threatening and more sinister than it was in the past. The biggest underlying shift here is the nature and purpose of viruses -- they are no longer created by kids trying to make a name for themselves… It is very targeted, specific criminal commercial based activity," said Scollay.

Michael Warrilow, director of Sydney-based analyst firm Hydrasight, agrees that although the ratio of virus laden e-mails has fallen, the overall threat has increased.

"[Administrators] are having some success at stopping some of these things at the front door but the fight is not yet over. [Malware authors] are moving onto new attack vectors -- things like instant messaging. [The threats] started off as a nuisance but will continue to evolve and get more targeted and more sophisticated," said Warrilow.

Earlier this year, Eugene Kaspersky, founder of anti-virus firm Kaspersky Labs, told ZDNet Australia that virus authors were choosing to infect fewer computers because they had less chance of getting caught by authorities.

Another reason to target a small group of users rather than the whole Internet population, according to Kaspersky, was that anti-virus companies would take longer to find samples of the virus and create a signature.

MessageLabs' Scollay said these targeted attacks are making life difficult for traditional signature-based anti-virus desktop applications.