People with herpes may have a higher risk of developing dementia later in life, according to scientists.
An estimated 700,000 Britons have dementia
They found that being infected with three strains of the virus may double a person's chances of being senile.
The risk applies to herpes simplex type 1, which causes cold sores, and herpes simplex type 2, which is transmitted through sexual contact.
They also implicate cytomegalovirus - a common strain that doesn't cause problems in healthy people.
It affects between 50% and 85% of people by the age of 40.
It can cause prolonged fever and mild hepatitis but it doesn't usually have long term health implications.
Dr Timo Strandberg and colleagues at the University of Helsinki based their findings on a study of 383 people.
They had an average of 80. Eight out of 10 had been diagnosed with heart disease and one in three had suffered at least one stroke.
They were all tested for each of the three strains of herpes.
Previous studies have suggested that there may be a link between these viruses and dementia.
A total of 106 people were found to be carrying antibodies to three of the viruses, which means they had been infected at some point in their lives.
A further 229 were found to have been infected with two of the three viruses. The remaining 48 had been infected with one or none of the viruses.
The researchers found that people who had been infected with two of these viruses were 1.8 times more likely to have dementia compared to those who carried one or none of the viruses.
People with all three viruses were 2.3 times more likely to have dementia.
The scientists suggested that the viruses could cause inflammation in the brain, which has been linked to dementia.
"Inflammation has been implicated in dementia and viral infections could be a triggering factor," said Dr Strandberg.
He said further research is needed to back up the findings but suggested they could help in the fight against dementia.
"Our findings should be tested in other studies but if these viruses are involved, there are existing therapies such as vaccination and antiviral drugs that could be used to prevent or treat dementia."
Those involved in the study were also tested for Chlamydia pneumoniae, which causes lung diseases, and Mycoplasma pneumoniae, which can cause a mild form of pneumonia.
Other studies have suggested that these too bacteria could be linked to dementia later in life.
However, the scientists said their tests showed no such link.
"Either bacteria are not involved or we could not discern their effects," said Dr Strandberg.
The study is published in Stroke: Journal of the American Heart Association.