The Melissa computer virus, discovered only last Friday, has spread itself by e-mail around the world and is causing havoc for large corporations, according to security experts.

Microsoft products are affected

"The virus is all over the place, across the globe," said David Emm, Total Virus Defence product manager at the UK's Dr Solomon, now owned by Network Associates.

"It's just a macro virus [ a common virus affecting small programs automating tasks in Microsoft Word] but it has this additional feature of being able to get around quick," he told BBC News Online.

The BBC's Chris Nuttall reports on how the virus spreads

As the Macro code is easily changed, a number of variants are emerging, notably the Papa virus, which affects Excel spreadsheets.

Melissa conceived in sex group

Dr Solomon's VirusPatrol, which checks newsgroups on the Net for developing viruses, discovered Melissa in a sex newsgroup on March 26.

The New York Times reported that several major US corporations shut down their mail servers on Friday night as they became overloaded with messages created by the virus.

Comments inside the macro virus identify it as "Melissa...by Kwyjibo." Computers become infected when users receive a particular e-mail and open a Word document attached to it.

The e-mail is headed: "Important Message From " and contains the sentence: "Here is that document you asked for...don't show anyone else ;-)." the attachment is usually called list.doc.

Scrabble virus's high score

If the user launches the document, their computer becomes infected, although the worst thing that can happen apparently is if it is launched when the day equals the minute value...such as 29 minutes past on the 29th, the following message appears:

" Twenty-two points, plus triple-word-score, plus fifty points for using all my letters. Game's over. I'm outta here."

The quote is from Bart of The Simpsons cartoon show, who invents the word Kwyjibo to describe a North American ape or his father Homer in a Scrabble-playing episode.

Graver implications of the virus are for company and Web servers carrying the huge volumes of e-mail being created.

When the document attachment is launched, a program is created which replicates the e-mail and sends it to the first 50 addresses in the Global Address Book of users running Microsoft's Outlook personal organiser.

Microsoft, Network Associates and other anti-virus and computer security companies have issued warnings and are supplying fixes for Melissa. But with many workers logging on to their computers still unaware of the virus, it was still expected to spread rapidly.