HUDDERSFIELD Royal Infirmary has been hit by a computer virus which has affected the entire network.
The system has been in crisis for five days after being hit by the Nachi virus - also called Welchi - which is spread from one computer to another.
Staff have been unable to find X-rays and test results have not been passed on at the right time.
A hospital insider, who did not want to be named, said: "Three working days, plus a weekend, seems a long time for a supposedly secure network to be in crisis.
"Numerous staff have had great difficulty doing their jobs.
"Patient details are throughout the network and the confidentiality of the system should be questioned."
A spokesman for Calderdale and Huddersfield NHS Trust, which runs the hospital, said: "The computer network at the trust has been disrupted by the international Nachi virus, that is affecting millions of IT systems across the world.
"The nature of this virus is that it generates a large volume of e-mail traffic, which causes computers to crash," he added.
"The virus has caused certain clinical IT systems to slow down. Alternative measures have been put in place to maintain these services.
"The IT department has been working round the clock to update all PCs and servers and ensure that all machines are updated in the form of a patch that will protect them from the virus."
The Leeds Teaching Hospitals NHS Trust, which includes St James' and the General Infirmary, was also badly hit by the virus a few days ago.
Mick Burgoyne, a Huddersfield-based computer expert, said organisations could have the best anti- virus software available, but often they were contaminated by links to external sites.
"The bigger hospital trusts should be able to set up good defences.
"But the difficulty is that they may connect with an awful lot of other people."
Mr Burgoyne said hospital trusts may have computer links to suppliers, with automatic ordering.
"That's where a weakness could come in," he said.
"Once a virus is in it can spread like wildfire and take a lot of work to remove."
Mr Burgoyne said viruses had an easier access to computer systems through broadband internet connections offered for home use.
"A lot get in through laptop computers, which people take home," he added.
"Unless people have a personal firewall, their computer can be open to attack.
"They can then go into work and plug in the portable - and the whole system is infected."