Video report by ITV News Science Correspondent Alok Jha
Alzheimer's could be transferred between one person and another during surgery, a new study suggests.
Scientists say the "seeds" of the disease could be transmitted through contaminated surgical tools and other medical procedures.
The study - published in the journal Nature - is the first evidence of dementia transmission in humans via microscopic protein fragments.
Until now it was thought the disease was brought on as a result of old age and genetic influences.
It has raised questions over the safety of certain medical procedures - including dental treatments and blood donations.
Charities and health officials are now calling for more research.
Researchers made the unexpected discovery whilst researching another brain disease, "iatrogenic" Creutzfeldt Jakob Disease (iCJD) - a brain-destroying condition known to be spread by contaminated surgical instruments and procedures.
Scientists said the research was still at an early stage and stressed that people could not "catch" the disease like a cold.
Researchers inspected the brains of eight patients who died from the disease after receiving a growth hormone extracted from dead bodies.
Six of the patients were found to have a hallmark of Alzheimer's - sticky clumps of fragmented protein called amyloid beta that form among nerve cells and on the walls of blood vessels.
In four cases, the amyloid deposits were widespread and only one patient was not affected at all.
Like the abnormal infectious proteins found in CJD, amyloid beta protein fragments stick to metal surfaces and resist conventional sterilisation.
Lead scientist Professor John Collinge, director of the Medical Research Council Prion Unit at University College London, said there was increasing evidence that neurodegenerative diseases including Alzheimer's might - in rare circumstances - be "acquired".
"You could have three different ways you have these protein seeds generated in your brain," he explained.
"Either they happen spontaneously, an unlucky event as you age, or you have got a faulty gene, or you've been exposed to a medical accident. That's what we're hypothesising."
But he said the public should not be alarmed as more research was needed.
Chief medical officer, Professor Dame Sally Davies, said there was "no evidence" that Alzheimer's disease can be transmitted in humans through any medical procedure.
"This was a small study on only eight samples," she said.
"We monitor research closely and there is a large research programme in place to help us understand and respond to the challenges of Alzheimer's."
What has been discovered?
- The first clinical evidence that "seeds" of brain abnormalities closely associated with Alzheimer's disease can be transmitted between people through medical procedures
How was the discovery made?
- Scientists from the Medical Research Council Prion Unit in London, were examining the brains of eight individuals who had died from Creutzfeldt Jakob Disease (CJD), a rare and lethal brain condition, who had received a pituitary growth hormone to treat stunted growth
- Unexpectedly, as well as the damage caused by CJD, researchers found signs of beta amyloid in six out of eight of the patients
What does this mean?
- Questions remain over how the protein got there. The eight patients were aged 36 to 51, very young to show clinical signs of Alzheimer's.
- After careful investigation, the scientists concluded the most likely explanation was that it was passed to the patients receiving the growth hormone treatment.
Why the concern?
- There is growing evidence that in rare circumstances it might be possible to "acquire" Alzheimer's, or at least the brain changes associated with it
Does this mean you can catch Alzheimer's?
- No, not in the sense that you can catch a cold - Alzheimer's is definitely not contagious
- But the possibility cannot be ruled out that brain changes associated with the disease can be triggered by microscopic proteins spread by some medical procedures
Could going to the dentist put you at risk?
- Almost certainly not
- But beta amyloid particles are known to stick to metal, and are hard to remove even by normal sterilisation procedures
- That means they could conceivably stick to surgical instruments, including the tooth probes used by dentists