A new study has discovered a connection between gut bacteria and Parkinson’s disease.

The incurable motor-neuron disease, symptoms of which include tremors and stiffness, was long believed to be caused by a buildup of protein in the brain. But a study published Thursday in the journal Cell suggests the disease could begin in the gut.

Parkinson’s patients typically experience digestive issues, such as constipation or bloating. These symptoms often precede the motor deterioration by years, which led researchers at the California Institute of Technology to search for a link between the seemingly unrelated conditions.

The Cal Tech team, led by Dr. Sarkis Mazmanian, took fecal samples from the patients and transplanted them into mice with Parkinson’s symptoms — which soon worsened. Mice that received samples from healthy patients remained the same.

“This was the ‘eureka’ moment,” said Dr. Timothy Sampson, a member of the research team. “The mice were genetically identical, the only difference was the presence or absence of gut microbiota.”

As much as 70 percent of neurons outside a person’s brain live in the intestines, and are connected to the brain through the central vagus nerve. The findings showed that irregularities in the gut’s bacteria could contribute to, or even cause, Parkinson’s.

“What we extrapolate from that is that there is a microbial profile that is different in Parkinson’s,” said Mazmanian. He noted that even though this was the case in mice, they’re still a “long way” from proving it’s the same in humans.

Approximately 60,000 Americans are diagnosed with Parkinson’s each year. Up to 1 million Americans live with the disease, and more than 10 million are affected worldwide. Researchers hope the study’s findings will pave the way for new treatments.