Five people in the UK are believed to have developed Alzheimer's Disease as a result of medical treatment decades earlier, doctors have revealed.
The world-first research suggests the individuals acquired a rogue protein that causes the form of dementia from growth hormone taken from the brains of deceased people.
The doctors say Alzheimer's itself is not transmissible and there is no ongoing public health risk because growth hormone given to children is now made synthetically.
At least 1,848 people in the UK were treated with cadaver-derived human growth hormone (c-hGH), between 1959 and 1985, most commonly to increase their height. Growth hormone is produced in the pituitary gland at the base of the brain.
It was withdrawn following evidence that some batches were contaminated with infectious prion proteins that caused the degenerative brain disease CJD in some people.
Doctors have now revealed clinical details of eight people referred to the National Prion Clinic at University College London Hospital. All had been treated with c-hGH as children, often over several years, according to the report published in the journal Nature Medicine.
Five had symptoms of dementia and another had mild cognitive impairment. Symptoms started when they were aged between 38 and 55.
The researchers say their unusually young age suggests they did not have so-called 'sporadic' Alzheimer's found in older people. And genetic testing ruled out an inherited form of the disease.
Professor John Collinge, who led the research and heads the University College London Institute of Prion Diseases, said it's likely the people all acquired the amyloid-beta protein that causes Alzheimer's through the hormone injections.
"The patients we have described were given a specific and long-discontinued medical treatment which involved injecting patients with material now known to have been contaminated with disease-related proteins," he said.
"There is no suggestion whatsoever that Alzheimer's disease can be transmitted between individuals during activities of daily life or routine medical care."
Read more from Sky News:
Fiona Phillips believes early starts may have brought on Alzheimer's
Hope for Alzheimer's blood tests on the NHS within five years
1.7m 'may be living with dementia in England and Wales by 2040'
'Findings pave the way'
There have been no reported cases of Alzheimer's acquired from any other medical or surgical procedures.
Nor is there is any suggestion that amyloid-beta can be passed on in day-to-day life or during routine medical or social care.
Co-author Professor Jonathan Schott, from the Institute of Neurology and the chief medical officer at Alzheimer's Research UK, said: "There is no risk that the disease can be spread between individuals or in routine medical care.
"These findings do, however, provide potentially valuable insights into disease mechanisms, and pave the way for further research which we hope will further our understanding of the causes of more typical, late onset Alzheimer's disease."
Prof Bart De Strooper, group leader at the UK Dementia Research Institute at UCL, said: "No one should reconsider or forego any medical procedure, especially for blood transfusion or neurosurgery which saves many lives worldwide every year.
"However, it is always important that we continue to review and scrutinise evidence where public health is concerned."