July 9 (UPI) -- 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 Tuesday in the journal JCI Insight.

Parkinson's disease is a neurogenerative disease that affects more than 1 million people in the U.S. While some cases are caused by genetics, the cause is unknown in most.

Parkinson's may be due to environmental causes. One million people have the neurodegenerative disease that robs them of their mobility. For some, the cause is genetic. For most, it's unknown. A team of Chicago area scientists sought to determine what triggers Parkinson’s.