Science is learning more and more about the bacteria and viruses that make their home in human bodies and the scope of their effects.

The billions of bacteria that live on and in our bodies, collectively dubbed the human microbiome, seem to influence every aspect of our physical and mental health. There’s a human virome, too. Little is known about the collection of viruses that naturally inhabit our bodies, but they also seem to affect a mind-boggling array of health conditions and behaviors.

A virus that normally affects freshwater algae, for one, causes minor cognitive impairment when it colonizes humans, a study just published in Proceedings of the National Academy of Sciences suggests.

“There’s more and more studies showing that microorganisms in your body have a bigger influence than anything anyone would have predicted, and this could be something along those lines,” said the paper’s senior author, James Van Etten, a University of Nebraska-Lincoln plant pathologist who first identified the virus, called chlorovirus ATCV-1, more than 30 years ago.

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Traces of chlorovirus ATCV-1, commonly found in freshwater lakes, turned up in the throat swabs of more than 40 percent of the participants in the study. Volunteers who had traces of the virus living in their throats performed slightly worse on tests of cognitive function than those who did not, even though there were no differences in education level or age that would account for the lower scores.

Specifically, those whose swabs showed traces of the virus performed worse on tests of visual processing and visual motor speed.