It is the unthinkable crime - the caring doctor who turns killer. But the truth is that while worldwide many patients have died at the hands of healthcare staff, few charges have been brought against doctors.
A research paper, Serial Murders in Health Care Institutions, found that more than a dozen healthcare workers were found to have murdered at least 170 patients in the past 20 years.
1The findings revealed a preponderance of nurses and nursing assistants as offenders. But the researchers said other health professions were also likely to have their share of such individuals.
And the true number of murders, what they described as 'the dark figure of crime', remained unknown.
Their work was prompted in part by the conviction of nurse Beverly Allitt in 1993 for the murder of four children at Grantham and Kesteven Hospital, Lincolnshire, and the subsequent inquiry.
Dr Cameron Stark, consultant in public health medicine at Highland health board and one of the authors, says coverage of the Allitt case seemed to suggest it was being treated as a one-off.
'We already knew of a number of cases of healthcare staff who had murdered those in their care. What irritated me about the Allitt report was that they dismissed all the previous cases.
'They said there had only been a few and they were not relevant. It seemed to me that they were incredibly relevant.'
One common feature identified in the study was that staff, who had serious suspicions, had repeatedly expressed them - but no-one had believed them.
Dr Stark suggests that clinical governance and a climate that allows staff to blow the whistle offer the best opportunities for prevention. While the aim will be to pick up poorly performing professionals, 'once in a blue moon you might identify that somebody is doing something deliberately', he says.
Professor Christopher Cordess, consultant forensic psychiatrist at Rampton Special Hospital, has interviewed several healthcare staff who have killed patients.
He is frequently called upon to give talks and advise occupational health staff on personnel screening but says he is never asked about doctors. However, he thinks this may change in the light of the Bristol baby deaths inquiry and doctors may find themselves more open to scrutiny.
When television filmmakers were researching the case of a Victorian doctor, William Palmer, who killed family members and some of his creditors, they approached Professor Cordess for advice. He steered them towards looking further than a financial motive.
'The trouble with all these things is they only go halfway to explaining why someone should use their position of power to turn the world upside down.'
Munchhausen syndrome by proxy should be considered, he says.
'It is like the fireman who lights the fire and gets a medal for being first at the scene. It is doing something more exciting, it is a risk thing, producing crises, bringing people back from the brink, living on that cusp of life and death.'
Affair to remember: Hawley Harvey Crippen Put the words doctor and murder together and the first name to spring to mind is Dr Crippen.
But the Crippen case entered the annals of criminology not because of the nature of the crime but because of the nature of the way the killer was brought to justice.
US-born Hawley Harvey Crippen was working in London and living with his second wife when he fell in love with his secretary, Ethel le Neve. Using poison, he killed his wife and dismembered her body, burned the bones and interred her remains in the cellar of their house in Holloway, north London.
Crippen then concocted an elaborate story to explain his wife's absence. He told her friends she had run off to the US and that she had died there. Her disappearance was unsuccessfully investigated by the police, but Ethel, who had by that time moved in with Crippen, lost her nerve.
The couple fled to Antwerp and, disguised as Mr and Master Robinson, they boarded an Atlantic liner. The ship's captain had heard about the Holloway investigation and became suspicious.
He contacted Scotland Yard by radiotelegraphy - the first time it had been used in a police investigation. The couple were arrested and tried at the Old Bailey and Crippen was executed in 1910.
Three-way mystery: Dr Robert George Clements Several doctors before and since Crippen have killed their wives, usually to clear the way for them to take up with a new lover. Such was the case of Dr Robert George Clements, only on a much grander scale. After having had three wives - all of whom died in suspicious circumstances but without any criminal charges being brought - Clements began courting Amy Victoria Barnett. In 1940, Vee, as she was known, was left£22,000 when her father died unexpectedly. The couple married and Clements returned to the lavish style he had become used to.
Seven years later, Vee was admitted to a nursing home in a coma and died the next morning. A post-mortem was carried out by Dr James Houston, but not before Clements had mentioned the possibility that his wife had died of myeloid leukaemia. Houston found what he thought to be myeloid leukaemia in the blood samples and put this on the death certificate.
But the rumours about Clements increased and the police ordered the funeral service to be stopped. On a visit to Clements' house the undertaker found him unconscious and he later died from a huge overdose of morphine.
The second post-mortem on Vee revealed traces of morphine.
When the police searched Clements' flat they found dozens of bottles of tablets.
On 2 June 1947, Dr Houston was found dead in his laboratory, having taken 300 times the lethal dose of sodium cyanide. The inquest on Vee had turned into a triple inquiry.
After 45 minutes the jury returned a verdict that Amy Victoria Clements was murdered by Dr Robert Clements.
Houston was found to have taken his own life while the balance of his mind was disturbed. On Clements' death, the jury had two possible verdicts - either suicide while of unsound mind or the rarer felo de se - self-murder to avoid the consequences of one's actions. The jury decided on the latter. The police did not reopen the cases of Clements' three previous wives.
Caught by his own folly: Thomas Neill Cream Dr Thomas Neill Cream stood in the dock at the Old Bailey on 17 October 1892 on charges of poisoning four prostitutes with strychnine. He was also charged with demanding money with menaces and attempting to administer strychnine to a fifth person with intent to murder.
Although the court decided to proceed with only one murder charge, evidence from the other cases was allowed to be heard. It took the jury just 10 minutes to find him guilty.
Cream had been born in Glasgow in 1850 and had emigrated to Canada.
Years later, he returned to Britain, and between 1876 and 1878 attended medical lectures at St Thomas' Hospital, London.
But when he turned to murder he failed to keep a low profile. He persistently wrote to the authorities about the murders and at one point went as far as to point the finger of blame at a fellow lodger. At 9 am on Wednesday 16 November 1892 he was hanged.
Wartime horror: Marcel Petiot One of the most notorious killers was Frenchman Marcel Petiot, referred to as Doctor Satan in the book Medical Murders.
2He was guillotined on 25 May 1946 after being found guilty of 26 murders. In interviews with police he upped his own murderous toll to 63.
The details of his crimes are truly grizzly. Living in Paris during the Second World War, Petiot hatched a hellish plan.
Pretending to be the head of an underground escape group, he put the word out that he could get people safely out of France.
His victims were mainly Jewish. They were told to bring any valuables they wanted to take with them, a minimum of luggage and his fee.
There were many theories about how Petiot dispatched his victims, but it is certain that in each case he pocketed the fee and their lifetime savings, which they had hastily withdrawn ready to start a new life abroad. Other belongings were sent to Auxerre, where Petiot's brother lived.
Eventually, police were called to Petiot's house to investigate neighbours' complaints about the incessant smoke and smell from his chimney. To their horror they found a stove full of corpses.
The subsequent trial was difficult to take in because of the enormity of Petiot's crimes. But in his book, Killer Doctors, Colin Evans says there was one aspect that the court could not escape.
3'Court attendants had covered an entire wall with a mountain of luggage recovered from Auxerre. Those silent cases spoke with a greater eloquence than any prosecution witness.'
1 Stark C et al. Serial Murder in Health Care Institutions. Published as 'Counting the dead', Nursing Times 1997; 93(46).
2 Goodman J (ed). Medical Murders. Judy Piatkus (publishers) Ltd, 1991.
3 Evans C. Killer Doctors. Michael O'Mara Books Ltd, 1993.
4 Devlin P. Easing the Passage. Bodley Head, 1985.
5 Stewart J. Blind Eye: how the medical establishment let a doctor get away with murder. Simon and Schuster 1999.