Alzheimer's disease may be passed from one person to another via blood transfusions and medical accidents, new research suggests.
Researchers at University College London found that the proteins that cause dementia, called prions, stick to metal instruments and resist sterilisation.
The scientists concluded that the "seeds" of Alzheimer's disease can potentially attach to surgical instruments and pass from one person's brain tissue to another.
Theoretically, this means an individual can be infected with Alzheimer's disease. But because this form of dementia has an incubation period of around 40 years, the individual may not know they have Alzheimer's disease until old age.
The researchers stumbled on the finding while studying the brains of eight patients who died from a strain of Creutzfeldt Jakob Disease (CJD).
According to the NHS, CJD is a "rare and fatal condition that affects the brain. It causes brain damage that worsens rapidly over time".
The eight patients in the study had all developed CJD after being injected with a human hormone treatment as children to aid growth problems. The human hormone treatment was banned in 1985.
The researchers were surprised to find high levels of amyloid beta protein in four of the patients and small amounts in three others.
Amyloid beta protein is known to form among brain cells and stop them communicating with each other properly when a person has Alzheimer's disease.
There is no evidence that CJD triggers the build-up of the Alzheimer's-related protein, so the scientists concluded that the "seeds" of Alzheimer's disease may have been lurking in the hormone injection, just as the CJD itself was.
Although none of the CJD patients, who all died between the ages of 36 and 51, had genetic variants associated with early-onset Alzheimer's disease, the scientists believe they would have been likely to develop Alzheimer's disease if they had lived for longer.
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 disease, may be "acquired" in rare cases.
"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," he said, according to the Press Association.
However, Collinge was also keen to point out that Alzheimer's disease should not be considered "contagious" in the same way that we think about cold and flu. He urged members of the public to continue with any planned medical procedures.
Chief medical officer for England, Professor Dame Sally Davies, also reassured the public, maintaining 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. 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," she said.
"I can reassure people that the NHS has extremely stringent procedures in place to minimise infection risk from surgical equipment, and patients are very well protected."