The cause of any disease is two separate issues. One, the etiology, concerns how the individual acquired the illness and the pathogenesis concerns the abnormal processes in the body that produce the signs and symptoms of the disease. For example, the etiology of acne is skin oil and bacteria, while its pathogenesis is blockage of pores with resulting infection and inflammation. For PD, our understanding of the etiology is poor to fair and our knowledge of the pathogenesis is fair to good.