It is not completely understood what causes Parkinson’s disease. However, experts suspect that the development of Parkinson’s disease is a result of interaction between genes and the environment. Toxins that have yet to be identified are believed to build up in a person’s body over time and may trigger Parkinson’s disease in people who are genetically predisposed to the illness.

Possible environmental factors that have been associated with Parkinson’s disease include rural living, exposure to well water and participation in agricultural work. This has led some experts to conclude that pesticides and herbicides may contribute to Parkinson’s disease. However, it is important to stress that many people with Parkinson’s disease have not been exposed to any of these risk factors.

Viruses may be another potential source of Parkinson’s disease. In 1918, some people who developed encephalitis (brain disease) as a result of the influenza epidemic later went on to develop symptoms resembling Parkinson’s disease. Women in Taiwan are believed to have developed similar symptoms as a result of exposure to the herpes virus. Scientists have not established the precise link between viruses and the development of Parkinson’s disease.

Some research also suggests a link between Parkinson’s disease and damage caused by free radicals. Free radicals are molecules that damage parts of the cells. This damage is known as oxidative stress and is believed to play a part in the development of numerous diseases including Alzheimer's disease and some forms of cancer.

Several genes have been linked to Parkinson’s disease, including a gene called parkin that helps break down defective proteins inside neurons. An altered parkin gene may prevent the breakdown of these proteins. As these proteins accumulate, they are believed to contribute to the destruction of neurons. However, genetic mutations are believed to account for only a small percentage of all cases of Parkinson’s disease.

The risk of developing Parkinson’s disease increases as a person ages. The average age of onset is 60, according to the National Parkinson Foundation (NPF). Roughly 5 to 10 percent of patients develop the illness at age 50 or younger, according to the National Institute of Neurological Disorders and Stroke (NINDS). Gender also appears to be a risk factor. According to the NINDS, Parkinson’s disease occurs twice as often among men than women, although the reasons for this are unclear.

People with a family history of Parkinson’s disease – particularly a first-degree relative who has had the disease – have significantly higher risk of developing Parkinson’s disease. It is believed that between 15 and 25 percent of patients with Parkinson’s disease have a family history of the disease, according to the NINDS.