Parkinson's disease (PD) is the second most common neurodegenerative disorder after Alzheimer's disease. The prevalence (i.e., proportion in a population at a given time) of PD is about 0.3% of the whole population in industrialized countries.
PD is more common in the elderly and prevalence rises from 1% in those over 60 years of age to 4% of the population over 80.
The mean age of onset is around 60 years, although 5?10% of cases, classified as young onset, begin between the ages of 20 and 50. PD may be less prevalent in those of African and Asian ancestry, although this finding is disputed.
Some studies have proposed that it is more common in men than women, but others failed to detect any differences between the two sexes. The incidence of PD is between 8 and 18 per 100,000 person-years.
Many risk factors and protective factors have been proposed, sometimes in relation to theories concerning possible mechanisms of the disease, however none have been conclusively related to PD by empirical evidence.
When epidemiological studies have been carried out in order to test the relationship between a given factor and PD, they have often been flawed and their results have in some cases been contradictory. The most frequently replicated relationships are an increased risk of PD in those exposed to pesticides, and a reduced risk in smokers.