The brain-signaling chemical dopamine, one of many neurotransmitters in the brain, is produced by cells in the brain stem. These cells release dopamine, one of the neurotransmitters that help control muscle movement. The shortage of dopamine is caused by the degeneration of neurons in the area of the brain that governs movement. The degeneration of the neurons is progressive.

About 50,000 new cases are reported each year, according to the National Institute of Neurological Disorders and Stroke (NINDS).

It is rare among people younger than 40, and the incidence rises with age. It is slightly more common in men than in women, according to the NINDS.

About 70 percent of people with Parkinson's first experience a slight tremor in a hand or foot, according to the Parkinson's Disease Foundation. The tremor usually occurs when the muscles are at rest and relaxed, and usually appears on one side of the body. Later it may spread to the other side of the body, but to a lesser degree. Common symptoms as the disease progresses include slowness of movement; rigid or stiff muscles; a shuffling gait; and a stooped posture, according to the NINDS. Secondary symptoms include changes in speech; loss of facial expression (called "flat effect"); difficulty swallowing; drooling; dementia; sleep disturbances; depression; memory difficulties and slowed thinking; sexual dysfunction; fatigue and aching; and urinary problems.

It is difficult to diagnose in the early stages. No test currently exists that can diagnose Parkinson's. Usually, a neurologist makes the diagnosis by evaluating the symptoms and their severity, as well as the results of a neurological exam, according to the NINDS. A brain scan can help determine if a patient has Parkinson's or a disease with similar symptoms. Sometimes drugs used to treat Parkinson's are given to a patient to see if the symptoms improve. The only way to truly confirm a diagnosis is through autopsy, when microscopic brain structures called Lewy bodies can be seen. These bodies form in Parkinson's patients.

Although there is no cure for Parkinson's, the symptoms frequently can be improved with medication and sometimes with surgery. Treatment varies greatly from person to person and depends on whether the symptoms are mild, moderate, or severe. Levodopa is a medication that is often prescribed. It is converted to dopamine in the brain and helps replace the dopamine that the brain no longer produces. There are other drugs that act like dopamine in the brain, and others that keep dopamine from being destroyed so more of it is available. It is common for a person with Parkinson's to take more than one drug. Potential surgical treatments include implanting neurostimulators, pallidotomy, and transplanting stem cells.