The virus that causes cold sores could be linked to Alzheimer’s disease, according to claims made by researchers that have re-ignited the controversial debate over whether the disease is infectious.

Academics from international universities including Oxford, Cambridge, and Bologna have warned of the implications of the rejecting the idea that Alzheimer’s could be triggered by an infection.

In an editorial in the ‘Journal of Alzheimer’s Disease’, the researchers wrote of their “concern” that “one particular aspect of the disease has been neglected, even though treatment based on it might slow or arrest Alzheimer’s disease progression.”

They went on to cite the “many” human studies “implicating specific microbes in the elderly brain,” The Times reported.

The brains of patients with Alzheimer’s disease have been found to have higher than normal levels of chlamydia and herpes, which causes cold sores, they said.

Addressing the common belief that an accumulation of the beta amyloid protein in the brain triggers the condition, the researchers claimed that the build-up was in fact a “defence mechanism.”

The claims are likely to cause concern for the 850,000 people who have Alzheimer's and other forms of dementia in the UK, and their relatives who fear they too could develop the condition.

However, other researchers and dementia charities have cautioned against the suggestions.

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Prof John Hardy, Professor of Neuroscience, UCL, said: “This is a minority view in Alzheimer research,” adding that there has been “no convincing proof” that infections cause the disease.

“We need always to keep an open mind but this editorial does not reflect what most researchers think about Alzheimer disease.”

Dr James Pickett, Head of Research at Alzheimer’s Society said: “A large number of different microbes including viruses, bacteria and fungi have been found in the brains of older people - but there do appear to be more of them in the brains of people who have died with Alzheimer’s disease.”

However, he said that while such observations warrant further research, there is currently not enough evidence to prove that microbes cause Alzheimer’s.

He went on to reassure the public that there is no evidence that Alzheimer’s is contagious or can be passed from person to person like a virus.

Dr Simon Ridley, Director of Research at Alzheimer’s Research UK, said there is “some” evidence to suggest that infections could “ramp up” the immune system and contribute to the progression of Alzheimer’s.

But he added: “There isn’t conclusive evidence to suggest that a particular infectious agent or microbe could be directly responsible for causing the disease.