Nine other children collapsed but survived - some with permanent injuries - during a two-month period in 1991 at Grantham and Kesteven Hospital, Lincolnshire.
John Goldring QC, for the prosecution, said some suffered heart-attacks - very rare for children - and some suffered respiratory attacks that made them stop breathing. 'Nothing like it had happened before. Nothing like it has happened since,' he told Nottingham Crown Court.
'There was a criminal at large on ward 4 (the children's ward).'
Since her arrest, Beverley Allitt, 24, of Grantham, has suffered from the slimmers' disease anorexia nervosa. She was transferred to Rampton maximum security prison last August for medical treatment.
Mr Goldring told the court: 'From the beginning people worried. On Saturday February 23 a little boy called Liam Taylor died from a massive heart- attack. He was just seven weeks old. Nobody could understand why.'
The pathologist who examined Liam's body wanted to bring an independent expert, and as further deaths and collapses followed, consultants became increasingly worried.
'On Tuesday April 30 the hospital authorities did something which you may think is quite unprecedented. They called in the police,' Mr Goldring said.
It soon became apparent that it was a criminal inquiry and that there was no medical explanation, he said. The first case police examined concerned five-month-old Paul Crampton, who collapsed three times within six days in March.
Mr Goldring said a massive amount of insulin was found in the child's blood.
There was only one possible explanation - on Thursday 28 March 1991, someone had injected virtually a whole adult syringe of insulin into this baby, Mr Goldring said.
'There was no conceivable medical reason to have done so. Had he not received very rapid treatment he would have died.
'The medical mystery was now over. It was clear, incredible as it may seem, that there was a criminal at large on ward 4.'
Mr Goldring said a number of things about the person responsible were plain. His or her presence on ward 4 could not arouse suspicion. 'The person, the criminal, had to have ready access to Paul. The suspicions of the police, parents or medical staff could not be aroused.
'The person, the criminal, had to have access to a hypodermic syringe and had to know how to use it. The person, the criminal, had to have access to insulin. The task for the police was plain. They had to try and find the answer to the question - who was that criminal on ward 4?'
Mr Goldring said police started looking at hospital records, among them a ward notebook, an old diary, which was used to make rough notes concerning patients. But the police found that someone had cut out a number of pages covering the period of Paul's stay in hospital.
The jury was shown a chart that indicated who was on duty at any one time in the ward. Mr Goldring said: 'A chilling fact emerged. One has only to look at that document to see what the link was. There was no suspicious collapse when the defendant was not on duty.'
When police first questioned the defendant, she had with her a ward allocation book showing which nurses were allocated to which patients, and when. Mr Goldring said: 'Later that ward allocation book disappeared from ward 4. Someone took it. Who?'
After Ms Allitt's arrest on 21 May, he said police found the missing ward allocation book in a bag in her wardrobe at home.
Mr Goldring claimed she had injected one baby with potassium and in other incidents she may have placed her hand over the babies' mouth or nose or interfered with their oxygen supply, with 'dramatic consequences'.
He said the defence team had made it plain that it did not intend to rely on any defence based on the defendant's mental state.
He told jurors they must not let any speculation about the defendant's mental state influence their verdicts. 'You have simply to decide 'did she do these things?' - not 'why did she do them?' '
Mr Goldring said Ms Allitt had been on duty in the ward on 24 occasions when children collapsed or died. One other staff member had been present on 10 occasions and the rest even less.
Ms Allitt was a student nurse at the hospital and her training was due to finish on 15 February 1991.
She failed an interview for a job at the hospital and a subsequent interview on 13 February for a job as a registered sick children's nurse at the Pilgrim Hospital, Boston, Lincolnshire.
'She was told she needed more experience nursing very sick children,' Mr Goldring said.
A few days later she was offered a job as an enrolled nurse at the hospital until the end of September.
On 21 February, Liam, a seven- week-old baby, was admitted to the ward and Ms Allitt was allocated to look after him.
He was suffering from bronchitis and pneumonia and needed oxygen to help him breathe and antibiotics.
Mr Goldring said Ms Allitt recorded that he had become distressed after feeding that evening and vomited a large amount of thick fluid. But Mr Goldring questioned the accuracy of her note and said she had 'painted a very dramatic picture' to his parents.
'It was in terms of sickness shooting across the room, touching the far wall, of having to change her uniform because of it.
'She said that at home they, Mr and Mrs Taylor, would have lost him. He would have choked to death. She may have said that Liam had stopped breathing.
'No one seems to have been aware of this drama described by the defendant on the ward.'
Mr Goldring said what was clear was that Ms Allitt had been with him and he was very ill. He remained ill the next day, but there were signs of improvement by early evening.
'For much of the evening and the early hours of the morning the defendant was alone with Liam in room 4,' he said. Two alarms had been attached to Liam, one to sound if he stopped breathing and the other to monitor the amount of oxygen in his blood. But it was possible to disconnect or override the alarms and if that was done and the baby stopped breathing, no one would hear. While Ms Allitt was alone with Liam, his condition deteriorated and when a superior was called she was surprised to find a large globule of sputum in his chest area, the like of which she had only ever seen previously in adults.
Soon after 4am, when the defendent had again been left alone with Liam, he suffered a heart attack.
There was much surprise over his sudden collapse. One senior nursing officer thought it bizarre, and other nursing staff were astonished. There had been no sound from the alarms.
Mr Goldring said this was the first of many such occasions when a baby in the defendant's care suddenly and surprisingly collapsed.
Liam did not die immediately. An X- ray was taken soon after 6am which showed that his chest condition had improved. But unfortunately he had suffered irreversible brain damage and later that day a decision was taken to switch off his life-support machine.
Mr Goldring said leading pathologists were surprised to discover during a post-mortem examination that part of the muscle around Liam's heart had died. That was extremely unusual in a child but what made it even more striking was the fact that there was no sign of any heart disease.
'You may think there was no natural reason for Liam to have suffered this heart attack. Something was done to him. We don't know what it was.
'A drug or strange mixture of drugs may have been administered or a hand may have been placed over his mouth or nose. It is easy to do that to a little baby. Different things may have been done at different times.'
Mr Goldring said that Timothy Hardwick, 10, had been admitted on 5 March 1991, after suffering an epileptic attack at the special school he attended. He had a mental age of three or four months but at the time of his admission was responsive, and teachers were not greatly worried about him.
But later that day he suffered a complete collapse and his heart stopped beating; he died almost immediately.
Mr Goldring said one heart attack involving a seven-week-old baby was surprising but staff could not understand two attacks involving a baby and a young child.
The pathologist diagnosed an epileptic attack. But it was widely accepted that that diagnosis was plainly wrong, Mr Goldring said.
Kayley Desmond was not seriously ill when she was admitted on 3 March. She had a chest infection and was on ward 4 at the same time as Timothy. For some time Kayley appeared to get better. But on Friday 8 March an X-ray suggested that she was worse and she was put on a drip feed.
On the night of 10 March Ms Allitt was one of two nurses on duty in the children's ward. Kayley stopped breathing and an X-ray revealed air inside her body by her right arm. 'We suggest that on any sensible analysis, that air could not have got there naturally,' he said.
Mr Goldring said someone had deliberately injected her either with a syringe full of air or with something else, with air also in the syringe.
After a further collapse and suspected heart attack, Kayley recovered quickly when she was taken to a Nottingham hospital.
Five-month-old Paul was admitted to hospital on 20 March - when Ms Allitt was off duty - suffering from bronchitis. He improved quickly and was expected to go home soon by the time she was next on duty on 23 March. But Mr Goldring said he deteriorated inexplicably that afternoon, becoming cold, clammy and lethargic. Ms Allitt suggested a test to check his blood sugar level - a surprising idea because Paul was not a diabetic, Mr Goldring said. That test indicated nothing wrong, but a similar one a few hours later showed his blood sugar levels were very low.
'It's absolutely plain someone had given Paul insulin,' Mr Goldring said.
Paul suffered a second attack brought on by low blood sugar the next day. Mr Goldring said Ms Allitt told Paul's mother she had given him some medicine. But he said the medicine she named was not prescribed for him by the doctors and Ms Allitt made no note of giving it to him.
When she returned to duty on Thursday 28 March and was assigned to look after him, he collapsed with a 'massive' hypoglycaemic attack.
The trial was adjourned until today.
Beverley Allitt: the full list of charges that she faces
Beverley Gail Allitt, 23, of First Avenue, Grantham, Lincolnshire, is charged with murdering Claire Peck, 15 months, from Balderton, Newark, Notts; Liam Taylor, eight weeks, from Gonerby Hill Foot, Grantham; Timothy Hardwick, 11 years, from Beeston, Notts; and Becky Phillips, nine weeks, from Grantham. Eighteen charges of attempted murder or alternatively causing grievous bodily harm relate to Becky Phillips' twin sister Kate; Kayley Desmond, 14 months; Paul Crampton, three months; Yik Hung Chan, two years; Christopher Peasgood, eight months; Christopher King, nine months; Patrick Elsthone, seven weeks; Bradley Gibson, five years; and Michael Davidson, seven years.
She is also charged with two further counts of murder and grievous bodily harm, on August 4, 1991, while on police bail. They involve Dorothy Lowe, 79, a resident at a nursing home at Waltham on the Wolds, Leicestershire, and Jonathan Jobson, 16, from Orton Goldhay, Peterborough.
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