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.

To support their findings, the researchers exposed mice to chlorovirus ATCV-1 and saw that their performance on cognitive tests got worse. In the mice, the virus broke through the blood/brain barrier and changed the activity of some genes in the brain. The genes affect neurotransmitters, including dopamine, as well as inflammation.

Scientists can’t measure people’s response to the virus directly, but the researchers had previously found evidence of the virus in a post-mortem human brain, co-author Dr. Robert Yolken, a professor of pediatrics at Johns Hopkins University medical school, told Healthline.

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“We’re really just starting to find out what some of these agents that we’re carrying around might actually do. It’s the beginning, I think, of another way of looking at infectious agents — not agents that come in and do a lot of damage and then leave, like Ebola virus or influenza virus. This is kind of the other end of the spectrum. These are agents that we carry around for a long time and that may have subtle effects on our cognition and behavior,” said Yolken.

The findings are strange, but not entirely without precedent. The parasite Toxoplasma gondii and some herpes simplex viruses can also affect cognitive function.

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“The thing that’s different about what we found is that [chlorovirus ATCV-1] is something that we wouldn’t have suspected would actually have any effect on humans or animals,” Yolken said.

While the virus is common in lakes, it’s not just a question of swimmers and boaters being infected. If it were that easy to pick up the virus, a very different picture of its effects would emerge, according to Van Etten.

While researchers piece together how the virus hitches a ride on a human host, they can also explore how to marshal the microbiome and virome to help control challenging conditions like cognitive decline.

“It points us in a direction of looking to see if we can improve people’s cognition, their behavior, by changing the composition of their microbiome,” Yolken said.