Warnings are being issued about the rapidly spreading Bagle Windows worm.
The Bagle virus has tricked lots of people
The virus poses as a test message and so far has tricked people in 87 countries into opening it and helping it spread.
Bagle plunders address books for e-mail addresses and uses an in-built mail program to send itself to new victims.
It also tells its creator about an infected computer and tries to open a backdoor that a malicious hacker could use to take control of the machine.
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"We have seen over 60,000 copies of Bagle, and this number is rising at an alarming rate," said Paul Wood, chief security analyst at MessageLabs.
MessageLabs said that currently 1 in every 136 e-mails it was stopping was bearing the virus.
Mr Wood said the Windows virus was spreading quickly despite using crude techniques to manipulate people into opening it.
The Bagle worm can be spotted in e-mail inboxes as it always has a subject line that reads simply: "Hi"
If you get an e-mail that looks like this, delete it
The text of the message makes the message look like it is a test from technical support. Sometimes the attached file bears an icon that makes it look like it is a calculator.
Users' machines will only be infected if they open and run the attachment bearing the worm.
Like many other recent viruses, Bagle travels with its own e-mail software that help it despatch itself to all the addresses it steals from an infected machine.
Many people in Australia look like they have been caught out by the virus as many of the copies of Bagle caught by MessageLabs have originated there.
Security experts believe that Bagle is modelled on the notorious Sobig series of viruses as it bears many similarities with those malicious programs.
Like those viruses Bagle has an in-built timer that, in this case, will stop the worm spreading on 28 January.
As well stealing e-mail addresses Bagle also attempts to let its creator know when it has managed to infect a new victim. It also opens up a backdoor on infected machines that hands over control of that machine to the virus' creator.