Doctors think the head trauma Muhammad Ali suffered during his boxing career might have contributed to his Parkinson's disease.

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Potential Causes of Parkinson's Disease

­About 1.5 million Americans have Parkinson's disease and 60,000 more are diagnosed with it each year [source: National Parkinson Foundation]. Most people who get the disease are older than 60, but about 15 percent are diagnosed before age 50 [source: National Parkinson Foundation].

Although doctors don't know what exactly causes Parkinson's, they think it stems from a combination of genetic and environmental factors. In about 5 to 10 percent of patients, there is a strong family history of the disease [source: National Parkinson Foundation]. Parkinson's may affect the same generation (siblings), or two different generations (parent and child) in this small percentage of familial cases.

Other possible causes of Parkinson's (and conditions like it) include:

Toxins: Scientists have linked exposure to manganese, carbon monoxide, cyanide and some pesticides and herbicides with a higher Parkinson's risk [sources: National Parkinson Foundation, Mayo Clinic]. However, most people with the disease haven't been exposed to these substances.

­­A virus: In the early 1900s, people who came down with a form of the encephalitis virus became stuck in a trancelike state in which they couldn't move or speak. When neurologist Dr. Oliver Sacks treated these people with levodopa, the same medication used to treat Parkinson's, they briefly came "back to life." (These "Awakenings" were depicted in the 1990 movie starring Robin Williams.)

Head trauma: Boxer Muhammad Ali, one of the most famous Parkinson's sufferers, developed the disease when he was only in his 40s. Researchers have been studying whether trauma to the head may play a role in the development of the disease.

Structural problems: Strokes and hydrocephalus (fluid buildup in the brain) are two conditions that may lead to an increased risk of parkinsonism.