Doctors explain what happens in the brain of someone who has the disease.

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Muhammad Ali may be most famous person ever to have had Parkinson's Disease, but it's by no means a rare condition.

An estimated 4 to 6 million people around the world suffer from Parkinson's, 1 million of them in the United States. What's more, experts say that number is likely to rise as the population ages.

In fact, getting older is probably the most common risk factor, says Britt Stone, MD, a movement disorders neurologist at Baylor Scott & White Health in Round Rock, Texas.

Most Parkinson's cases strike after age 60, but 15% of cases are in people under 50, and 10% are in people younger than 40, according to the National Parkinson Foundation. This is called young onset Parkinson's; it's the diagnosis that actor Michael J. Fox received when barely 30 years old. Muhammad Ali was diagnosed in his early 40s, marking his case a relatively unusual one as well.

RELATED: Michael J. Fox discusses Muhammad Ali's impact on Parkinson's

Parkinson's also affects men slightly more than women, but otherwise is mostly an equal opportunity disease, affecting a range of races and ethnicities, as well as people in different regions and with different incomes.

Biologically, Parkinson's happens when the neurons that produce dopamine start to die. "Dopamine is a really important chemical for lots of things—mood, ability to move in an organized and predictable fashion," says Dr. Stone.

Lack of dopamine accounts for the "big four" symptoms of Parkinson's: tremor, stiffness in the limbs, slowness of movement, and problems with balance.

"There can be slowness of movement, which can look like a softer voice, slower walking, not swinging your arms when you walk," says Dr. Stone.

Patients might also have a slow blink rate, have trouble rolling over in bed, buttoning clothing, and their faces may not be as expressive as before.

Very little is known about what causes the dopamine neurotransmitters to die. "Twenty percent of patients with Parkinson's disease have a genetic condition that explains their disease," says Diego R. Torres-Russotto, MD, director of the Movement Disorders Program at the University of Nebraska Medical Center. "The other 80% are patients that get the disease and we are not sure why."

It's likely that environmental exposures can trigger Parkinson's in people who are already susceptible, he says. Many believe that pesticides, such as Agent Orange used during the Vietnam War, are a culprit.

There's also speculation that repetitive brain injuries—such as those sustained by Ali over his long boxing career—might be a risk factor for Parkinson's, but there's no real evidence to back this up.

Even though Parkinson's is usually not diagnosed until most of the cells that produce dopamine are damaged, people often live long productive lives after that. "The way the disease happens in every patient is different," says Dr. Torres-Russotto. "The progression and the signs and symptoms vary a lot among different individuals with Parkinson's disease."

Parkinson's has no cure, but there are treatments, including medications to boost dopamine levels in the brain. Some people also benefit from deep brain stimulation, when electrodes are implanted into the brain.

Lifestyle factors, especially physical activity, can make a big difference in quality of life for people with Parkinson's, says Dr. Stone.

"Our goal is to make Parkinson's Disease be the kind of thing that's in your head, but in the back, not dictating your day-to-day life," she says.

And Muhammad Ali is one great example of that.