Spam is overtaking viruses as the biggest pain for businesses using the web.

Monthly reports for March from antivirus companies show that, while virus activity is experiencing single digit growth, spam is growing at between 10 and 30 per cent and now accounts for one in every 2.8 emails.

"Spam certainly is getting to be the major issue," said Paul Wood, information analyst at MessageLabs.

"Whereas a virus infection costs a lot they tend to be isolated occurrences. Spam is like death by a thousand cuts, once you factor in the cost of bandwidth, storage and administration.

"Analysts are predicting that half of all emails will be spam by the end of the year. MessageLabs reckons that will happen within the next three months."

Over one in 400 spam emails monitored by the company had pornographic content. The most common attachments were .JPG, with .GIF files coming in third.

However, predictions that the outbreak of hostilities in Iraq would bring a flood of virus activity from the Middle East have failed to materialise.

Although the Ganda worm did use the war as a social engineering technique it was relatively ineffective.

"We've seen a little rise but none of the deep cyber-terrorism that gets people worried," said Jack Clark, analyst at Network Associates.

"Most of the activity has been based around website defacements and denial of service attacks. The virus writing community seems to be sitting this one out."

But there is also little evidence that businesses are updating antivirus definitions as often as they should.

Klez is still the most common virus and has topped the charts all year.