More than one million people in the United States are living with Parkinson’s disease, and each year, about 90,000 people are diagnosed with it. While some cases are linked to genetics, most are not — and the cause has remained a mystery. But research from Northwestern Medicine may have uncovered an unexpected clue.

A 2025 study found that a usually harmless virus, human pegivirus (HPgV), was present in the brains of people with Parkinson’s disease — but not in those without it. This discovery could point to a possible environmental trigger for the disease.

What Is HPgV?

HPgV is a blood-borne virus in the same family as hepatitis C. It’s common and is not known to cause any disease.

“We were surprised to find it in the brains of people with Parkinson’s at such high frequency and not in the controls,” says Igor J. Koralnik, MD, chief of neuroinfectious diseases and global neurology at Northwestern Medicine. “Even more unexpected was how the immune system responded differently, depending on a person’s genetics.”

How the Study Worked

Dr. Koralnik and his team, including Barbara Hanson, PhD, who was the lead author on the study, used a tool called ViroFind to analyze brain samples from people who had passed away. They studied brains from 10 people with Parkinson’s and 14 people who died of other causes (the control group).

HPgV was found in the brains of 5 out of 10 people with Parkinson’s, but in none of the control group. It was also found in the spinal fluid of people with Parkinson’s, but not in others.

“HPgV is a common, symptomless infection previously not known to frequently infect the brain,” Dr. Koralnik explains. “This suggests it could be an environmental factor that interacts with the body in ways we didn’t realize before.”

Dr. Hanson adds, “Having found this virus in particular in the brain and at such a high rate of frequency was a substantial finding.”

What the Virus Might Be Doing

Dr. Koralnik’s study showed that people who had HPgV in their brains also had higher levels of tau (a protein that can build up and damage brain cells) and complexin-2 (a protein involved in brain signaling). These changes are often seen in diseases that affect memory and movement, suggesting that HPgV may play a role in Parkinson’s-related brain changes.

Scientists also looked at blood samples from more than 1,000 participants in the Parkinson’s Progression Markers Initiative, a project launched by The Michael J. Fox Foundation.

“With the blood samples, we observed similar immune-related changes, mirroring those found in the brain,” Dr. Koralnik says. “People who had the virus showed different signals from the immune system than those who didn’t, and this pattern was the same, no matter the genetics. But as we followed each person over time, we saw a more complicated picture.”

A Gene Mutation May Play a Role

The study found that people with a certain Parkinson’s-related gene mutation, LRRK2, responded differently to the virus compared to others with Parkinson’s but without the mutation.

“We plan to look more closely at how genes like LRRK2 affect the body’s response to other viral infections to figure out if this is a special effect of HPgV or a broader response to viruses,” adds Dr. Koralnik.

What’s Next?

Going forward, the research team plans to study more people to find out how common HPgV is in people with Parkinson’s and whether it plays a role in the disease.

“One big question we still need to answer is how often the virus gets into the brains of people with or without Parkinson’s,” says Dr. Koralnik. “We also aim to understand how viruses and genes interact — insights that could reveal how Parkinson’s begins and could help guide future therapies.”

As scientists continue to explore the link between viruses and Parkinson’s, this discovery opens new doors for understanding — and possibly treating — the disease.