The 'lovebug' computer virus forced a major London teaching hospital to shut down everything except 'core critical clinical systems'.
Guy's and St Thomas' Hospital trust was hit by an 'infestation' of 500,000 'I love you' e-mails carrying the virus last Thursday.
Head of information technology and telecoms Jenny Nash described it as 'year 2000 five months late', as the trust fell back on plans it had made to circumvent the millennium bug.
But most other NHS organisations claimed to have been largely unaffected.
Some admitted this was because they do not use Microsoft's Outlook software. And Oxfordshire health authority IT manager David Britton said: 'We were lucky - our NHSnet link had gone down' when the virus struck.
An NHS Information Authority spokesperson said NHSnet had to be taken down 'for three-and-a-half hours while 30,000 virus messages were cleared from the messaging service'. It faxed warnings to HAs.
Catriona Bowker, head of IT at St Mary's trust, south London, said it had avoided the virus through 'good fortune, good virus protection and good warnings from the NHS'.
But an HSJ source warned that the NHS's downloadable 'address book service', with '6,000 or 7,000' e-mail addresses, left it vulnerable to viruses like the 'lovebug'.
The Information Authority spokesperson said 'the benefits [of the service] far outweigh any risks from viruses'.