That was true even when the researchers weighed other factors that affect older adults' mental sharpness, including education, smoking, heart disease and diabetes.
But the study hinted that exercise might play a protective role. The research team found that infection "burden" was related to mental impairment only among sedentary people -- and not those who said they got some exercise.
However, that too needs to be studied further, the team noted.
Katan said that infection with the viruses, rather than the two bacteria, seemed to play a greater role in mental decline. Overall, 23 percent of the study participants had signs of mental impairment at the study's start; the odds of impairment were 2.5 times higher among people who carried all three viruses -- HSV 1 and 2, and cytomegalovirus -- than for people who carried only one virus.
"It's not just infection with one virus," Katan said. "It's the overall burden."
Infections alone would not be the whole story, either, said Dr. Timo Strandberg, a researcher at the University of Helsinki in Finland, whose own work has uncovered a link between herpes viruses and dementia.
Most people carry at least one of these pathogens; up to 80 percent of U.S. adults are infected with cytomegalovirus by age 40, for example. But not all develop dementia.
"Cognitive [mental] decline in old age is multifactorial," said Strandberg, who co-wrote an accompanying journal editorial.
For example, the gene variant APO E4 makes people more vulnerable to developing Alzheimer's disease, and one theory has been that if infections can promote mental decline, APO E4 carriers would be more susceptible.
There was no evidence of that in this study, however.
Strandberg agreed that more research is needed to know whether herpes viruses or other infections really contribute to mental decline.
In this study i
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