Three hospitals in London are reported to be infected by a variant of the Mytob worm.
According to the BBC, St Bartholomew's (also known as Barts) in the City, the Royal London Hospital in Whitechapel and The London Chest Hospital in Bethnal Green have been forced to shut down their entire computer systems as a result of the infection.
A statement on the Barts website has attempted to reassure the public and patients that the attack was being dealt with and that no one was in any danger.
According to The Register, some doctors have resorted to using pen-and-paper backup systems.
The Mytob worm spreads via email, planting a backdoor Trojan horse which can be used by remote hackers to gain access and control over a victim's computer. The computer can then be spied upon (to steal confidential information), or used to send spam or launch denial of service attacks.
There will, no doubt, be concerns that the confidentiality of patients' data may have been put at risk and the hospitals will surely be keen to reassure the public that security has been maintained.