Cold sores may increase the risk of impaired memory and thinking skills, research suggests.
The herpes simplex virus that causes cold sores was one of a number of infectious agents associated with reduced mental ability.
Scientists tested the thinking and memory of 1,625 people from New York City with an average age of 69.
Participants had their blood tested for five common low-grade infections: oral and genital herpes, cytomegalovirus, the respiratory form of chlamydia, and the stomach bug Helicobacter pylori.
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Those with higher levels of infection were 25% more likely to deliver low score results in a standard mental function test.
Repeated tests over an average of eight years showed no sign of the effect worsening with time.
The findings appear in the latest issue of the journal Neurology.
Study leader Dr Mira Katan, from Columbia University Medical Centre in New York, said: "We found the link was greater among women, those with lower levels of education and Medicaid or no health insurance, and most prominently, in people who do not exercise.
"While this association needs to be further studied, the results could lead to ways to identify people at risk of cognitive impairment and eventually lower that risk. For example, exercise and childhood vaccinations against viruses could decrease the risk for memory problems later in life."