Hackers and spammers have "raised their onslaught" with two global email-borne virus attacks launched in December and January.

The attacks were so large that they drove up the level of viruses up by a factor of 20 compared with usual activity, according to data from security firm Postini.

The January attack became known as the Storm worm because the original email subject line was '230 dead as storm batters Europe'. At the time of the email, there was a heavy storm occurring in Europe.

The email that contained the virus frequently mutated to show dozens of different sensational but believable headlines designed to tempt the reader into clicking on an attachment and infecting their computer.

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