Alzheimer's disease may be passed on through blood transfusions and contaminated surgical instruments, scientists have suggested.
The brain disease may be transmissable in the same way as Creuzfeldt Jakob Disease (CJD), the human form of mad cow disease, researchers at University College London speculated in a small study.
However, the Alzheimer's Society of Ireland last night urged caution in interpreting the results.
Tina Leonard, the organisation's head of advocacy said: "While the findings certainly sound alarming, there is currently no evidence that the disease is a contagious condition and can be transmitted from person to person via a medical procedure."
Her comments were supported by Brian Lawlor, Professor of Old Age Psychiatry of St James's Hospital, who said the study is small and that more work is needed.
Drawing conclusions at this stage is premature, he added.
The biggest risk factor for Alzheimer's is age, along with genetics and lifestyle factors such as smoking, obesity and high blood pressure.
The landmark finding, described as a "paradigm shift", suggested it was possible that the "seeds" of dementia could be transferred from the brain tissue of one person to another.
They said the proteins that cause dementia are a type called prions which can stick to metal surfaces, like surgical instruments, and are resistant to conventional sterilisation.
They suggested it could be theoretically possible to become infected with Alzheimer's seeds through a blood transfusion, brain surgery, or invasive dental work, like a root canal operation.
And because the incubation period can be up to 40 years, people could be unaware that they have been contaminated.
The scientists stumbled on the discovery while studying the brains of eight people who died of CJD. All had developed the disease after being injected with human growth hormone taken from bodies between 1958 and 1985, when the practice was banned.
Unexpectedly, four of the patients had huge levels of amyloid beta protein - a sticky deposit which forms among brain cells and stops them communicating with each other properly in Alzheimer's patients.
Smaller amounts were found in three others. Although none had developed dementia, scientists say it is likely they would have, had they lived longer.
"What we need to consider is that in addition to there being sporadic Alzheimer's disease and inherited or familial Alzheimer's disease, there could also be acquired forms of Alzheimer's disease," said lead scientist Prof John Collinge.
"What relevance this has to common forms of Alzheimer's disease out there, we don't know," he added.
The study is published in the journal 'Nature'.