A common virus once thought harmless to humans might be linked to Parkinson’s disease, a new study says.

The germ, Human Pegivirus (HPgV), was found in half the autopsied brains of patients with Parkinson’s, but not in any brains from healthy people, researchers reported July 8 in the journal JCI Insight.

“HPgV is a common, symptomless infection previously not known to frequently infect the brain,” lead researcher Dr. Igor Koralnik, chief of neuroinfectious diseases and global neurology at Northwestern Medicine in Chicago, said in a news release. “We were surprised to find it in the brains of Parkinson’s patients at such high frequency and not in the controls.”

The virus also appeared to prompt different responses from people’s immune systems, depending on their genetics, Koralnik said.

“This suggests it could be an environmental factor that interacts with the body in ways we didn’t realize before,” Koralnik said. “For a virus that was thought to be harmless, these findings suggest it may have important effects, in the context of Parkinson’s disease. It may influence how Parkinson’s develops, especially in people with certain genetic backgrounds.”

Parkinson’s disease occurs when brain cells that produce an important hormone called dopamine begin to die off or become impaired.

As dopamine levels decrease, people develop movement symptoms like shaking or stiffness, as well as problems maintaining balance and coordination.

More than 1 million people in the U.S. live with Parkinson’s disease, including actors Michael J. Fox and Alan Alda, singer Neil Diamond and football great Brett Favre. About 90,000 new cases are diagnosed every year, researchers said.

Most cases of Parkinson’s are not linked to a person’s genetics, raising the question of what might trigger the death of dopamine-producing nerve cells, researchers said in background notes.

For the new study, researchers autopsied the brains of 10 Parkinson’s patients and 14 people not suffering from the disorder.

The team found HPgV in 5 out of 10 brains from people with Parkinson’s, but none of the 14 healthy brains. The virus also was present in the spinal fluid of Parkinson’s patients, but not in the control group.

Further, more brain damage was found in patients with HPgV, researchers said.

Next, researchers tested blood samples from more than 1,000 participants in the Parkinson’s Progression Markers Initiative, a biosample library available for Parkinson’s research. HPgV is a blood-borne virus in the same family as hepatitis C.

Only about 1% of Parkinson’s patients had HPgV in their blood samples, researchers found.

But people who had the virus showed different signals from their immune system, particularly those with a Parkinson’s-related gene mutation called LRRK2, researchers said.

“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,” Koralnik said.

Researchers plan to continue tracking how common the virus is among Parkinson’s patients, and how it might trigger the brain disorder.

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