IFNA1 and Parkinson disease: Another set of studies found an increased incidence of PD amongst those chronically infected with hepatitis C virus (HCV), but these studies are confounded by the fact that some HCV patients received interferon (IFN) treatments; with a follow-up investigation finding a much stronger relationship between IFN-treated groups and PD (249 PD incidents/100000 person-years) than non-IFN-treated groups (30 PD incidents/100000 person-years) [46–48].