More than one million people in the United States have Parkinson disease, more than are diagnosed with multiple sclerosis, Amyotrophic lateral sclerosis (ALS, known as Lou Gehrig's disease), muscular dystrophy, and myasthenia gravis combined. Parkinson disease affects about 1 in 100 Americans over the age of sixty years. The average age of onset is about sixty, and while the illness is less common at younger ages, it does occur. The exact number of cases in younger individuals is difficult to determine, but it is estimated that five to ten percent are diagnosed prior to age forty.

Getting an accurate count of the number of people living with Parkinson disease is difficult because many people in early stages of the illness assume that their symptoms are the result of normal aging or other health problems, such as arthritis. Early Parkinson symptoms are subtle and sometimes transient. Often it is the spouse or other relative who first notices slight abnormalities in gait or movement. Diagnosis is sometimes difficult and uncertain because other conditions have symptoms that mimic those of Parkinson disease. It is important to seek a referral to a neurologist who is knowledgeable about movement disorders to confirm the diagnosis and address concerns of the patient and family.

Parkinson disease affects men and women in almost equal numbers, with a slight predominance in males at younger ages. It knows no social, ethnic, economic, or geographic boundaries. Some demographic studies have shown that African-Americans and Asians are less likely than Caucasians to develop Parkinson disease; the reasons for this variance are unclear.

Parkinson disease does not shorten one’s life expectancy, although there is currently no cure. It is also not contagious (cannot be passed from one person to another by casual or intimate contact).