Companies including Microsoft, Intel, the merchant bank Merrill Lynch and some British banks were said to be affected. But only hard-disk files running Microsoft's Windows system were hit. Robert Carrigan, a spokes- man for Merrill Lynch, one of the biggest American merchant banks, said the program struck its New York offices on Thursday. "We worked through the night over here to ensure that protective measures were put in place," he said. The British subsidiary was almost unaffected.
Spokesmen for other major businesses said they simply closed off their e-mail systems and implanted electronic safeguards.
Computer experts describe Worm Explore as a "worm" rather than a "virus" because it passes itself from computer to computer, rather than multiplying within the same computer. It propagates by electronically raiding the infected user's e-mail address book and sending out copies of itself to people listed there, while wiping the data from files picked randomly from any part of the internal network the computer was attached to.
"We have the virus," said Rachel Albert, a spokeswoman at InterActive Public Relations in San Francisco, where the worm hit early on Thursday. "It's terrible. A lot of people lost everything they were working on." British companies with American offices were particularly at risk.
Victims got an e-mail apparently replying to one they had sent, with an attached file and the message: "Hi... I have received your e-mail and I shall send you a reply ASAP. Till then take a look at the attached zipped docs. Sincerely... " Opening the attached file restarted the destruction and again attacked the e-mail address book.
The Melissa worm appeared in March, sending 50 copies of a list of pornography sites on the Web to people. An estimated 100,000 computers were affected worldwide, as e-mail systems overloaded with the volume of traffic.
Such viruses and worms are becoming increasingly common, as virus-writers realise the potential of the internet and office e-mail practice for spreading their destructive products.
They are helped by the enormous popularity of Microsoft's Windows system, and programs including Word for word processing, Outlook Express for e- mail, the spreadsheet program Excel and presentation system PowerPoint. At present, commercial virus checking programs scan for more than 41,000 viruses of various forms that can affect computers using Microsoft programs.
Ironically, the trial yesterday in which the US Department of Justice claims Microsoft breaks anti-trust rules by using its operating system dominance to push its own software heard evidence that Microsoft's actions make it harder for companies to protect themselves against viruses.
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