There has been a substantial reduction in the number of global computer virus epidemics, with the 38 serious outbreaks that occurred in the first half of 2004 falling to just two since last June.

Eugene Kaspersky, technical director at antivirus firm Kaspersky Lab, which compiled the figures, said: "On the good side we have seen police arresting more hackers - over 100 in the past year - and in some cases they have been getting long sentences. A group of local hackers who attacked a Chinese bank got 15 years for their crimes, which is very good."

But the shift from amateur virus writers to organised criminal gangs has changed the method and effect of attacks, according to Kaspersky.

Bank account phishers, for example, do not need viruses spreading worldwide for kudos; they can get good money from just a few thousand infections that steal bank account details.

Spammers have also been using viruses, and only need a few thousand consumer PCs or a handful of company systems to send out large quantities of spam.

There is also disturbing evidence that hackers and virus writers are working together to trade stolen email addresses, Kaspersky warned. Emails stolen by one spamming gang's Trojan software were soon being used by other gangs, indicating that the groups are in regular and frequent contact.

Hacking is also becoming increasingly lucrative, as more of the world's systems go online. One organised hacking group in Brazil cleared more than £30m last year, according to Kaspersky.