Hall of Fame quarterback Brett Favre disclosed Tuesday that he has recently been diagnosed with Parkinson's disease. That puts him among the 1 million people nationwide who are living with the disease, according to the head of the Parkinson's Foundation.

John L. Lehr, CEO of the Parkinson's Foundation, said in a statement after Favre's testimony that 90,000 people in the United States will be diagnosed with Parkinson's disease (PD) this year alone. Parkinson's is a progressive nervous system disorder that affects movement.

"Scientists believe that PD is caused by a combination of genetic and environmental factors," Lehr said. "Environmental factors may include head injuries, pesticide exposure, or area of residence."

Actor Michael J. Fox is among the most recognizable advocates for awareness and research into Parkinson's. In sports, boxer and activist Muhammad Ali, who died in 2016, made his diagnosis public in 1984 three years after retiring from boxing. Other sports figures with the disease include former NBA player Brian Grant; American cyclist Davis Phinney; former MLB outfielder and manager Kirk Gibson; and former NHL coach and longtime hockey analyst Barry Melrose, who retired from ESPN in October 2023 after being diagnosed. Former New York Giants and New York Jets punter and broadcaster Dave Jennings died in 2013; and former NBA coach Jerry Sloan died in 2020.

Parkinson's is a nervous system disorder that so far has no known cure. Symptoms generally begin with mild tremors, stiff or jerky muscles, or decreased ability to move quickly or automatically. Patients in the early stage of the disease also may experience depression, anxiety, fatigue and memory issues, according to the Mayo Clinic.

The disease gradually deteriorates the nerve cells in patients' brains, particularly in an area called the substantia nigra, which produces dopamine, a chemical that -- in addition to heavily impacting mood -- acts as a messenger between the nervous system and the brain.

Scientists have yet to determine the exact cause of this cell degeneration. Studies have uncovered several potential factors, including rare genetic predispositions and exposure to some chemicals, including herbicides and pesticides. The condition affects more men than women.

In addition to helping the brain control the body through the nervous system, dopamine also plays a role in aiding memory, sleep and concentration. It's most well-known for regulating mood and giving the brain feelings of motivation, satisfaction and general pleasure. In addition to physical symptoms, Parkinson's patients may report sleep disorders and cognitive decline.

Because the cause of Parkinson's has yet to be discovered, medical experts have no sure way of preventing the disease. However, some studies have linked regular aerobic exercise and caffeine consumption as activities associated with lower risk. Symptoms generally don't appear until about 80% of the affected nerve cells have deteriorated, according to the American Association of Neurological Surgeons.

The average age of patients at diagnosis is 60, according to Johns Hopkins, and diagnoses to people under 50 are rare. At 54, Favre is relatively young but not unprecedentedly so.

However, having multiple concussions -- which Favre has reported suffering on the field -- has been linked to an increased risk of Parkinson's. A 2020 study published in the journal Family Medicine and Community Health found that a single low-impact concussion increased a patient's risk of developing Parkinson's by 57% and that multiple concussions can compound the danger.

Dr. Michael Okun, a neurologist at University of Florida Health, said multiple concussions and those received in short succession can more than double the risk of developing Parkinson's later in life, along with other issues caused by continual brain injuries. Okun treated Muhammad Ali and noted that the late boxer's career put him at serious risk.

Chronic traumatic encephalopathy (CTE) is also associated with Parkinson's. Although the two conditions function differently -- unlike the dopamine-attacking enzymes in Parkinson's, CTE is characterized by tau proteins -- there are similarities in cause and in effect.

"They're slightly different, but there's clearly overlap in some of the symptoms, and in terms of the risk factor of having repeated concussions or other head trauma," physician and medical journalist Dr. Céline Gounder told ESPN.

But doctors are even more concerned about the chemical risks. Some toxins have been linked to Parkinson's risk far beyond what's been observed in patients with histories of multiple concussions, Okun said, including those used in dry cleaning and landscaping. If environmental factors contributed to a football player's diagnosis, the pesticides sprayed on football fields could be as or more relevant to the disease as the roughness of the game.

When Favre testified to congress Tuesday, he characterized a concussion drug treatment as "too late" to help him. While there is no known cure for Parkinson's, recent medical advances have successfully staved off the worst symptoms in some patients for months or years. Common tactics employed by neurologists include several medications aimed at blocking the enzymes that break down what dopamine the brain does produce, as well as medicines to help control involuntary jerking and tremors.

Doctors may also prescribe carbidopa-levodopa, which is a combination of chemicals that enters the brain and converts to dopamine. However, the effectiveness can deteriorate over time and leave patients seeking other options. One such treatment is dopamine agonists, which instead of actually becoming dopamine in the brain instead serve to mimic its effects.

"When you hear those four words, 'You have Parkinson's disease,' that shouldn't be the end of the conversation," Okun told ESPN. "There are so many great treatments for this, and we have folks that are living with Parkinson's disease for 10, 20, 30 years -- living well."

Favre had taken the stand before the House Ways and Means Committee to testify about welfare reform. Prevacus, a company working on a medication to mitigate the effects of repeated head injuries, had received nearly $2 million in welfare money. Prevacus' founder, Jacob VanLandingham, pleaded guilty in July to wire fraud after Mississippi state audits revealed that some of the welfare money he had earmarked for the drug actually went to cover gambling debts. Documents show that Favre invested over $800,000 into the company and lobbied government officials for more funding. He was Prevacus' top investor.