Avoiding Nimda

Be wary of unexpected blank e-mail messages with long subject lines and attachments If you receive a message like this delete it without reading it If you use Outlook with Windows 98 or Windows 2000, install Microsoft's security patch that stops viruses like Nimda Install anti-virus software and keep it up to date If you have been infected, use clean-up programs from anti-virus companies to expunge it from your computer

The highest concentration of infected systems was in Canada, Denmark, Italy, Norway, the UK and the United States.

But it may take some time before the full extent of the damage caused by Nimda is known, as many computers may still be infected but be cut off behind firewalls.

Experts fear the attacks could prove to be more widespread than the Code Red infections of July and August, which caused an estimated US$2.6 billion in damage.

Business 'ground to a halt'

A British businesswoman who e-mailed BBC News Online said the worm had had a devastating effect on her website.

"The computers were rendered inoperable by the fact that many thousands of infected files were being created, and deleting them and deleting the Recycle Bin seemed to make them replicate faster," said Sarah Mabbitt of Avidstitch. "The entire hard disks and memory were used up.

"My business has ground to a halt and I do not yet know what damage has been done to vital files," she said.

The worm first appeared in the United States on Tuesday and spread to Asia overnight. Thousands of European businesses began work on Wednesday with infected computer systems.

The worm, the name of which spells admin backwards, arrives in electronic mail without a subject line and contains an attachment titled "readme.exe" disguised as an audio file.

It also scans webpages for e-mail addresses and sends a message to that site with a copy of the worm attached.

It can also interrogate copies of a program called Microsoft Exchange that many companies use as a "post office" for the e-mails and messages of their staff.

The worm can also copy itself to any shared directories it finds on networks it has compromised.