Security experts say they have contained the Sobig worm by identifying and blocking computers that are key to co-ordinating it.
China has been hit especially hard by "Sobig"
Computer specialists cut off the worm's circulation by repairing 20 servers it was using to blast infected emails into users'
inboxes around the world.
However, in China the virus was reported to be spreading very quickly, with one internet security firm in Beijing suggesting that 30% of computers may have already been infected.
Initial fears over an instruction in the worm's code to point infected machines to a server controlled by the virus writer between 1900 and 2200 GMT (2000-2300 BST) on Friday were unfounded.
Another instruction will repeat the request on Sunday at the same time and experts have again expressed concerns that a malicious program could be downloaded, but fears have largely subsided.
"There's a potential risk for Sunday, but I think it's really mitigated," said Chris Rouland, vice president for research and development at US-based Internet Security Systems.
"All the network operators are aware they need to block these [servers the worm is expected to point to] now."
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Some analysts fear the worm will instruct computers to download a file that could launch another virus or spam attack,
collect sensitive information, or delete files stored on an infected computer or network.
Friday's instruction resulted in infected machines being re-directed to pornographic websites.
The worm generated so much email that many inboxes are now completely full and are bouncing back new messages, which is only
adding to the problems Sobig is causing.
Net giant AOL said it had stopped more than 23 million copies of the virus and email filtering firm MessageLabs said it had
caught more than 3.5 million.
The worm has appeared in numerous forms with the Sobig F virus being the sixth variant of the malicious program which first
appeared in January.
Security experts say the virus was probably written to order for spammers who want to find a way to spread their unwanted
commercial e-mails without fear of being traced.
Chinese web security firm, Beijing Rising Technology, said on Friday the Sobig F Worm had infected 30% of all e-mail
users in the country.
This equates to some 20 million users in a country second only in its number of surfers to the United States.
The BBC's correspondent in Beijing, Francis Marcus, says there is scepticism over this figure, but added analysts agreed
China's 68m Internet users had been "highly vulnerable" to attack.
Key reasons for this are firstly that only 60-70% had installed anti-virus software, according to Beijing Rising
SOBIG SUBJECT LINES
Re: my details
Re: That movie
Re: wicked screensaver
Re: your application
Secondly, rampant piracy of software in the country and elsewhere means packages are not updated as often as they are
intended to be because they are not commercially registered, our correspondent said.
"We've never seen anything like it," said Hao Ting, a spokeswoman for Beijing Rising Technology.
"People, who may not be totally aware of the danger, simply open their e-mail and don't discover it's a virus until it's too
China's National Computer Virus Emergency Centre describes the virus as "very serious."