An uncommon medical treatment withdrawn in the mid-1980s may have caused some very rare cases of Alzheimer's, scientists believe.
The team at University College London has been studying five cases linked to injections of human growth hormone that came from deceased donors.
The findings do not mean Alzheimer's is infectious - you cannot catch it from contact with people who have it.
But it might be "seeded" or transmitted into the brain by certain procedures.
The researchers say all of the people in their study had been treated as a child with cadaver-derived human growth hormone, or c-hGH, that was contaminated with brain proteins that are seen in Alzheimer's disease.
There is no suggestion it can be passed on in day-to-day life or during routine medical or social care. There is no ongoing public health risk because growth hormone treatment is now made synthetically.
The c-hGH was used to treat at least 1,848 people in the UK between 1959 and 1985.
Its use was stopped when experts recognised that some batches were contaminated with a different type of infectious protein which had caused a rare and fatal brain condition called Creutzfeldt-Jakob disease (CJD) in some people.
The latest findings in Nature Medicine suggest Alzheimer's-related amyloid protein might be spread accidentally during medical and surgical procedures in the same way as CJD.
The researchers stress that the circumstances are highly unusual - there have been no reported cases of Alzheimer's acquired from any other medical or surgical procedures.
And as c-hGH treatment is no longer used, there is no risk of any new transmission via this route.
Lead author Prof John Collinge, director of the UCL Institute of Prion Diseases, said: "There is no suggestion whatsoever that Alzheimer's disease can be transmitted between individuals during activities of daily life or routine medical care.
"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."
Dr Richard Oakley, associate director of research and innovation at Alzheimer's Society, said: "There is no cause for concern for the health of the general population."
Prof Bart De Strooper, from 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."
The Nature Medicine study reports on eight people referred to UCLH's National Prion Clinic at the National Hospital for Neurology and Neurosurgery in London, who had all been treated with c-hGH in childhood, often over several years.
Five of them had symptoms of dementia, and either had already been diagnosed with Alzheimer's disease or would otherwise meet the diagnostic criteria for this condition.
Another person met criteria for mild cognitive impairment.
These people were between 38 and 55 years old when they started having neurological symptoms.
Researchers say the unusually young age at which these patients developed symptoms suggests they did not have the usual Alzheimer's which is associated with old age.
