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Parkinson’s Disease is the second most common neurodegenerative disorder after Alzheimer's disease and affects approximately seven million people globally and one million people in the United States.[36][70][107] Parkinson’s Disease causes lung infections and heart disease, which in many cases leads to death because many patients are older and their bodies cannot fight off the infection. In a study done on those with Parkinson’s Disease in 1995, a mortality rate of 70% was concluded because of the disease and the infections it causes. The epidemiological measure of prevalence is used to describe the proportion of a population with a disease. Similarly, another epidemiological measure is incidence, which describes the appearance of a disease in a population over a set time period. These measures can be used to describe the overall severity of Parkinson’s Disease in relevant population context. Additionally, these measures can be adjusted to different lengths of time to further assess Parkinson’s Disease, normally used on an annual scale. Thus, the prevalence of Parkinson’s Disease is widely considered to be from 100 to 200 cases per 100,000 people, with an annual incidence rate of 15 new cases per 100,000 people. Specifically in North America, the prevalence is 572 cases per 100,000 people. Differences in continental prevalence against global prevalence is suspected to be explained by differences in screening among certain populations. The overall prevalence of Parkinson’s Disease varies dramatically from country to country. For example, the prevalence in China is 15 (per 100,100 population), whereas Argentina’s prevalence is much higher at 657 (per 100,000 population). The proportion in a population at a given time is about 0.3% in industrialized countries. In 2017, later research was conducted on Parkinson’s disease and an updated incidence rate ranged from 5 to less than 35 cases per 100,000 persons per year. Parkinson’s Disease is more common in the elderly and rates rise from 1% in those over 60 years of age to 4% of the population over 80.[70] The mean age of onset is around 60 years, although 5–10% of cases, classified as young onset Parkinson’s Disease, begin between the ages of 20 and 50.[10] This number has increased from past years in men over 70. The rate ratio, which is used to compare incidence rates to each other, has increased to 1.35 per decade with a 95% confidence  interval of 1.10-1.65  In other words, it is suspected that the incidence rate of Parkinson’s Disease in men over 70 has increased compared to previous findings, but it should be noted that this conclusion is debated due to the statistical significance of the study observing it. This is also disputed because of the variation of prevalence across the world. Some reasons for this variation are different survival rates and medical records using epidemiological studies. Males are more often affected than females at a ratio of around 3:2.[4] Within North America, the prevalence for Parkinson’s Disease among  women is 488 and for men it is 667 (per 100,000 population). However, while some studies have proposed that it is more common in men than women, others failed to detect any differences between the two sexes.[70] Parkinson’s Disease may be less prevalent in those of African and Asian ancestry, although this finding is disputed.[70] The number of new cases per year of Parkinson’s Disease is between 8 and 18 per 100,000 person–years.[70] An individual’s odds of developing Parkinson’s Disease increases 3.5 times if they have an immediate family member with the disease. In 5-10% of patients, a genetic factor is the cause of the disease. This genetic factor may be inherited or a genetic mutation.

Many risk factors and protective factors have been proposed, sometimes in relation to theories concerning possible mechanisms of the disease, however, most have not been conclusively related to Parkinson’s Disease by empirical evidence. When epidemiological studies have been carried out in order to test the relationship between a given factor and Parkinson’s Disease, they have often been flawed and their results have in some cases been contradictory.[70] The most frequently replicated relationships are an increased risk of Parkinson’s Disease in those exposed to pesticides, and a reduced risk in smokers.[70][108] One generally accepted exception is caffeine, where its consumption is found to be a protective factor against Parkinson’s Disease. Some other factors that are suspected to increase the risk of Parkinson’s Disease occurring in a person are dairy product consumption, a history of melanoma, and a traumatic injury to the brain. Some other factors that are suspected to decrease the risk of Parkinson’s Disease in a person are caffeine consumption, smoking, and physical activity. There have been some trials that have been investigating the effects of smoking and caffeine on decreasing the risk of the early onset of Parkinson’s Disease. Smokers are less likely to get Parkinson’s Disease due to the number of chemicals that are found in cigarettes and tobacco, which can have an influence on biological processes. Smoking leads to a reduction of certain processes in the brain, which is thought to protect the brain through inhibiting enzymes from oxidizing dopamine. Furthermore, physical activity is the main behavior that has been found to help prevent the initial development of Parkinson’s Disease, and can also help to decrease an individual’s risk of various other diseases.

Several diseases have been linked to increasing the progression of Parkinson’s Disease including diabetes, depression, and anemia. A 2007 study done on Finnish men and women with diabetes showed a 38% increase in the development of Parkinson’s Disease. However, this was not the only disease development associated with Parkinson’s Disease. Anemia has a pattern of being an early indicator of Parkinson’s Disease, sometimes starting up to 20 years prior to the disease itself. Anemia also is associated with the disease severity, meaning that lower levels of hemoglobin indicate more severe symptoms in Parkinson’s Disease.

Moreover, Parkinson’s Disease is frequently observed as one disease in patients with several comorbidities. Specifically, depression, either major or dysthymic, has been frequently seen in individuals with Parkinson’s Disease. Initially, some studies have reported the prevalence of depression in patients with Parkinson’s Disease as high as 63% in 1967. More recently in 1999, studies have adjusted that prevalence of depression to be 20-40% in these patients with Parkinson’s Disease. Ultimately, several studies have reported an increased rate of comorbidity with several mood and anxiety disorders along with Parkinson’s Disease.