Malware in the form of email attachments has dropped heavily in the last month, according to security software specialist Sophos.

In August there was an average of one infected message in every 1,000 emails, compared with one in 322 during the first six months of 2007.

Criminals are finding other, more effective ways of spreading malware, such as spam campaigns directing people to infected webpages, said Sophos senior security consultant Carole Theriault.

'Most malware writers seem to be taking an extended holiday from spreading their malicious code via email attachments, and are using spam and the web instead to infect users,' she said.

The total number of infected webpages continues to grow. During August, Sophos detected an average of 5,000 new infections each day.

Three quarters of the world's infected web pages are hosted in China, The United States or Russia, says Sophos.