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.

A progressive neurological disorder affecting 10 million people globally, Parkinson’s can cause disruptive symptoms such as tremors, immobility and loss of balance. Now, scientists at Northwestern Medicine have discovered that a usually harmless virus — the Human Pegivirus — may be a contributor to the disease.

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.