The chief medical officer for England, Professor Sir Liam Donaldson has told a conference of health experts more needs to be done to improve patient safety.
The majority of people visit hospitals without any problems
Experts from the UK and Europe gathered in Cardiff on Thursday for a conference run by the Welsh NHS Confederation.
Also speaking was the medical director of Glan Clwyd Hospital, which is running a patient safety pilot project.
It is estimated around 40,000 deaths a year in the UK are caused by "breakdowns in patient safety".
At the conference, managers discussed how to improve safety for the 8.5m people admitted to hospital in the UK annually.
Although a huge majority of people are treated safely, it is estimated that 400,000 "avoidable events" - or breakdowns in patient safety - happen each year.
These include medical errors, machinery failing and infections acquired in hospital.
The NHS pays out an estimated £400m to settle clinical negligence claims annually and it is thought hospital-acquired infections cost the NHS nearly £1bn every year.
Breakdowns in patient safety are thought to account for more deaths than accidents on the road and in the workplace combined.
Around 300 delegates from 27 countries attended the conference at Cardiff International Arena.
Speaking as the chairman of the World Alliance for Patient Safety, Sir Liam said that by reporting all incidents including near misses, hospitals can learn lessons.
Speakers also included Dr David Gozzard from Glan Clywd hospital in Bodelwyddan, which is one of four in the UK taking part the £4m Safer Patient Initiative pilot project organised by the Health Foundation.
Dr Gozzard said the scheme "had to be ambitious".
He added: "I think for a patient to come into hospital with one illness only to suffer from problems with that illness in another sphere - these are things which are preventable."
Welsh Health Minister Brian Gibbons said the Welsh Assembly Government's strategy for the Welsh NHS has patient safety at its heart.
He added: "Work is well under way in Wales to help improve patient safety.
"All hospital sterilisation and decontamination units across Wales have been upgraded and have now met tough EU standards."
Also on Thursday, the House of Commons public accounts committee said a "fog of ignorance" remained in the health service over the extent and cost of hospital-acquired infections.
The committee described figures of an estimated 300,000 infections picked up in healthcare settings as "rough and ready".
Meanwhile, Department of Health figures have shown that almost four out of 10 English NHS trusts had seen MRSA cases rise in the last year.