The epidemiology of PD has been greatly aided by two natural experiments that generated important hypotheses regarding its etiology. The first was the encephalitis lethargica epidemic, which suggested a role for an infective agent (20). The second was the strange occurrence of MPTP-induced parkinsonism (21), which suggested the role of a neurotoxic agent and led to studies examining the role of pesticides because of its similarity with paraquat (22). The relevance of these models for the etiology of PSP and/or MSA is far more questionable. However, in the absence of any other clues, most researchers have simply used risk factors that have been suggested to be important for PD, e.g., smoking behavior, head injury, pesticides, well water, etc., and tested them out in PSP and MSA as essentially a hypothesis-generating exercise.
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