It may, however, be overly simplistic to assume there is a continuous evolution from the inception to the full development of PD.44 Lewy body neurodegenerative processes may occur in a stepwise fashion, requiring additional causative factors to advance to the next level. In fact, such a noncontinuous evolution seems plausible, given the relative proportions of asymptomatic (incidental) Lewy body pathology vs PD in the general population. Incidental Lewy body disease is found in approximately 10% to 17% of people older than 60 years,36,45,46 which is 5 to 10 times the frequency of PD. Therefore, in some individuals, the pathogenetic process may not advance to the full clinical syndrome without a second (or more) causative factor(s). Thus, the epidemiologic search for the causes of PD must encompass early and later factors (ie, multifactorial and multistage causes).