We know that viruses can cause all kinds of illnesses—the flu, COVID-19, chickenpox—but what if a virus could alter your brain function without making you sick?
That’s exactly what researchers at Johns Hopkins University have discovered with a little-known virus called ATCV-1, a pathogen that was once thought to only infect algae.
Here’s where it gets eerie: This virus has been found inside the throats of healthy humans, and those who tested positive for it performed worse on cognitive tests than those who didn’t.
Infected individuals were slower at processing visual information, more easily distracted, and had shorter attention spans.
And it’s not just a human phenomenon—when scientists deliberately infected lab mice with ATCV-1, they observed similar cognitive declines.
The infected mice struggled with memory tasks, took longer to solve mazes, and showed signs of reduced attention span.
This raises a big, unsettling question: How many of us might be carrying this virus without even knowing it?
And what does it mean for our brain function?
A Virus That Wasn’t Supposed to Be in Humans
For years, ATCV-1 was believed to be harmless to humans, simply a virus that infected freshwater green algae.
It was large—much bigger than most viruses—and functioned in ways more typical of bacteria than a traditional virus. Scientists barely paid attention to it.
That changed when researchers unexpectedly discovered ATCV-1 in human brain tissue samples.
This raised an obvious but chilling question: How did it get there?
At first, the finding was dismissed as a coincidence.
The researchers couldn’t prove whether the virus had infected the brain before or after death, and without further evidence, they let it go.
Then, a new study changed everything.
The Discovery That Made Scientists Rethink Everything
A team led by pediatric infectious disease expert Robert Yolken decided to investigate further.
They examined 92 healthy adults living in Baltimore, checking for traces of ATCV-1 in their throats.
The results?
43% of them tested positive for the virus—nearly half.
That was shocking enough.
But what they found next was even more bizarre.
The Virus and Its Effect on the Human Brain
When the researchers compared cognitive test scores of infected and uninfected participants, a pattern emerged:
- Those with ATCV-1 scored about 10% worse on visual processing tasks.
- They were slower at connecting a sequence of numbers on a page—a simple test of attention and processing speed.
- They showed signs of shorter attention spans and increased distractibility.
While the difference wasn’t dramatic enough to be debilitating, it was consistent and statistically significant.
In other words, the virus seemed to be subtly affecting cognitive function.
Here’s the Evidence
Skeptical that a virus could influence brain function, the researchers decided to take their experiment further.
They infected lab mice with ATCV-1 and put them through a series of cognitive tests.
Just like in humans, the infected mice performed worse than their uninfected counterparts:
- They took 10% longer to navigate mazes.
- They spent 20% less time searching for objects.
- They showed clear signs of reduced attention and spatial awareness.
Michael Byrne at Motherboard summed it up bluntly:
“They had created stupid mice, it seemed, or at least mice that had a harder time with some tasks.”
These findings suggested that ATCV-1 wasn’t just coexisting with its human and mouse hosts—it was actively altering brain function.
Could Viruses Be Influencing Our Behavior?
For decades, scientists have believed that viruses only affect the brain when they cause severe diseases like rabies or encephalitis.
But what if common, symptomless viruses were subtly shaping our thoughts, memory, and behavior without making us sick?
This isn’t as far-fetched as it sounds.
Other microorganisms have been shown to influence animal behavior in surprising ways:
- Toxoplasma gondii, a parasite carried by cats, has been linked to risky behavior in humans and behavioral changes in rodents that make them more likely to be eaten by cats.
- The gut microbiome—trillions of bacteria inside us—has been found to affect mood, anxiety, and cognitive function.
Now, ATCV-1 joins the list of potential brain-altering microbes.
What’s Happening in the Brain? The Genetic Clues
To dig deeper, the researchers examined the brains of the infected mice. They focused on the hippocampus, the region crucial for memory formation and spatial awareness.
What they found was shocking:
- The virus altered the activity of nearly 1,300 genes in the hippocampus.
- These genes were linked to learning, memory, and immune responses.
- The virus itself didn’t seem to live inside the brain, but it may have been triggering immune reactions that led to cognitive changes.
According to neurobiologist Mikhail Pletnikov, who worked on the study:
“The similarity of our findings in mice and humans underscores the common mechanisms that many microbes use to affect cognitive function in both animals and people.”
This suggests ATCV-1 might not be directly attacking the brain—but it could be interfering with immune responses, leading to brain changes as an unintended side effect.
What Does This Mean for Us?
If a virus we’ve never heard of can subtly alter cognitive function, it raises big questions:
- How many other viruses are affecting our brains without our knowledge?
- Could common infections play a role in conditions like ADHD, depression, or memory decline?
- Should we be testing for brain-altering viruses as part of routine health screenings?
At the moment, ATCV-1 doesn’t appear to be harmful enough to warrant immediate concern—but the fact that it can subtly influence cognition means we may need to rethink the role of viruses in human health.
Could this virus be affecting your cognitive function?
There’s no way to know—yet.
A New Frontier in Brain Science
The discovery of ATCV-1’s effects on cognitive function opens up a new frontier in neuroscience and microbiology.
It forces us to consider an unsettling possibility:
We might not be entirely in control of our own minds.
If a virus from algae can slightly lower attention and processing speed, what else is lurking in our bodies, subtly shaping our thoughts, behaviors, and intelligence?
One thing is certain—this is just the beginning.
With further research, scientists might uncover even more hidden influences on the brain, leading to new treatments for cognitive disorders—or even ways to counteract the effects of brain-altering viruses.
For now, you might want to be a little more careful around algae-filled lakes.
Who knows what else is hiding in the water?
