In a landmark finding described as a "paradigm shift", researchers at University College London said it was possible that the "seeds" of dementia could be transferred from the brain tissue of one person to another.
Worryingly, the proteins causing dementia are a type called prions which can stick to metal surfaces, like surgical instruments, and are resistant to conventional sterilisation.
It means that it would 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.
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 Professor John Collinge, director of the Medical Research Council Prion Unit at UCL.
"You could have three different ways you have these protein seeds generated in your brain. 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. It's a paradigm shift.
"What relevance this has to common forms of Alzheimer's disease out there, we don't know. Could a small percentage of these cases be related to seeds from the environment?"
Previous experiments on laboratory mice and monkeys had already shown transmission of the Alzheimer's protein is theoretically possible.
The scientists are confident the amyloid beta deposits were not caused by CJD. Brains of 116 patients with prion diseases who had not received pituitary growth hormone did not have the Alzheimer's hallmark. Writing in the journal Nature, the study authors conclude it was likely infectious Alzheimer's proteins were passed at the same time as CJD.
"Alzheimer's protein seeds could follow similar transmission pathways," added Collinge. "The seeds will potentially stick to metal surfaces whatever the instrument is. With prions, we know quite a lot about that. Certainly, there are potential risks with dentistry where it's impacting on nervous tissue, for example root canal treatments.
"If you are speculating that amyloid beta seeds might be transferred by instruments, one would have to consider whether certain types of dental procedure might be relevant." Although the risk is low, the researchers said that determining whether the proteins could be passed through medical instruments and metal surfaces should be a research priority.
However the authors urge people not to be concerned about planned medical procedures, and to dismiss any notion of Alzheimer's being "contagious" in the same way as flu.
"No way is this suggesting that Alzheimer's is a contagious disease," he said.
"You can't catch it by living with someone who has Alzheimer's disease or being a carer. No one should consider cancelling or delaying any kind of surgery. But I think it would be prudent to do some research in this area."
Professor Roger Morris, Professor of Molecular Neurobiology, King's College London, said: "This is a landmark paper in providing evidence, for the first time in man, of a mechanism for the propagation of Alzheimer's disease that we already know exists from experimental studies in mice."
However, health experts said that the risks were extremely low and people should not be overly concerned by the findings.
Paradigm shift
• Alzheimer's is the most common form of dementia, accounting for around 62 per cent of all diagnosed cases.
• Dementia is a loose term used to describe different degenerative disorders that trigger a gradual loss of thinking, remembering and reasoning.
• In most people, symptoms first appear in their mid-60s. The disease is progressive and incurable.
• There are treatments to reduce symptoms. Scientists believe genetic, lifestyle and environmental factors all play a role in causing it.
