Brett Favre has Parkinson's disease. What is it, and how common is it among former NFL players?

(This story was updated to add a new photo gallery.)

Former Green Bay Packers great and pro football Hall of Famer Brett Favre revealed on Tuesday that he has been diagnosed with Parkinson's disease.

Favre shared the news during a congressional hearing in Washington, D.C. It's unclear how long he has been suffering from the disease, but he said during the hearing his diagnosis was recent.

Here's a short summary of what Parkinson's is and entails.

What is Parkinson's disease?

Parkinson's disease is a degenerative brain condition most often associated with slowed movements, tremors, balance problems and more, according to the Mayo Clinic and other health organizations.

Most causes are unknown, but some people can inherit the disease. While Parkinson's is most known for affecting movement and balance, it can have wide-ranging effects on cognitive ability, mental health and on a person's senses.

What are the causes of Parkinson's?

Research has shown certain risk factors, such as prolonged exposure to pesticides, are linked to Parkinson's, but the causes for the disease are largely unknown. About 10% of cases are genetic with the disease being inherited from one or both parents.

Experts believe non-genetic causes for Parkinson's are linked to the buildup of unusable proteins in certain cells. Proteins are chemical molecules that have a very specific shape. When some proteins don’t have the correct shape — a problem known as "protein misfolding" — your body can’t use them and can’t break them down, according to the Cleveland Clinic.

The buildup of these so-called Lewy bodies causes toxic effects and cell damage.

What are the symptoms of Parkinson's?

The best-known symptoms of Parkinson's involve the loss of muscle control, but experts now believe the disease can also affect non-motor functions in the body.

Common movement-related symptoms include slowed movement, tremors (especially while a muscle is at rest), rigidity or stiffness and an unstable posture or walking gait.

Non-movement related symptoms include the loss of sense of smell, constipation, sleep problems, and trouble thinking and focusing.

Is there a cure for Parkinson's disease?

There is no cure and it is estimated that nearly one million Americans are afflicted with the disease.

Is Parkinson's disease common among former NFL players?

Some research has linked participation in American football to higher odds of having a Parkinson's diagnosis.

One study, published in 2023 in the Journal of the American Medical Association, found that longer duration of football play, and a higher level of play, were associated with higher odds of having Parkinson's or "parkinsonism," which is a set of movement symptoms associated with Parkinson's disease and other disorders.

According to Boston University researchers, 90% of former NFL players have shown signs of the degenerative brain disease chronic traumatic encephalopathy (CTE), which is caused by frequent concussions.

In a 2015 landmark settlement, the NFL promised to compensate former players who developed dementia and other brain diseases tied to concussions. Parkinson's is a disease covered by that settlement.

USA Today contributed to this report.

This article originally appeared on Milwaukee Journal Sentinel: What is Parkinson's disease? Causes, symptoms, Brett Favre diagnosis