Computer systems at three London hospitals remained down on Wednesday morning after PCs were apparently infected with malware.
No patient data was at risk of disclosure, said William Mach, an NHS spokesman. As a precaution, computers were shut down at St Bartholomew's, the Royal London Hospital and The London Chest Hospital.
When the infection became known, ambulances were diverted to other hospitals, as it was easier to admit patients using unaffected computer systems rather than revert to a paper-based admission systems, Mach said.
The hospitals are now taking emergency patients again, he said.
Official are investigating how the infection occurred, although it did not appear to be malicious, Mach said.
"We've got a team in place that is desperately trying to restore full power to everyone," he said. "They are preoccupied with trying to get the systems online."
The PCs were running antivirus software, Mach said. Other media reports said the computers were infected with the mass-mailing worm Mytob, discovered in early 2005.
Once running on a PC, Mytob collects e-mail addresses and e-mails itself to other PCs as an attachment. Mytob, also known as MyDoom, can also download other harmful software. However, most antivirus products can detect and remove it.
Hackers, however, often use variety of methods to repackage their malicious software using methods such as compression to make a file appear different and catch antivirus products off guard.
McAfee said in a statement it was given a sample of the version of Mytob affecting the NHS earlier on Tuesday. The company said its products have detected it since Jan. 13 and that up-to-date products would detect and clean an infection.
"McAfee is only able to comment on the threat itself and any questions regarding the NHS' specific use of McAfee technology should be raised with the NHS," the statement said.
Mach said an internal team has been created to deal with the infections. So far, the NHS has not alerted the police.
"It is not yet known yet how it infected systems in the first place," Mach said.
The National Health Service uses an e-mail system, NHS mail, which contains the contact details for hundreds of thousands of NHS staff.
According to a NHS Web site, the system "is protected by cutting edge anti-virus and anti-spam protection."
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