What Is Parkinson’s Disease?

The retired N.F.L. quarterback Brett Favre told a congressional committee on Tuesday that he had been diagnosed with Parkinson’s disease, a progressive, incurable condition that causes tremors and stiffness, often resulting in impaired movement and speech.

The Republican-led committee was hearing testimony related to a sprawling welfare scandal in Mississippi in which millions of federal dollars earmarked for needy families went instead to the wealthy and powerful, including Mr. Favre.

Here’s what to know about the diagnosis, symptoms and treatment of the condition, which affects at least 1 percent of people over 60.

Parkinson’s disease affects the central nervous system. It causes nerve cells in the brain to weaken and eventually die, leading to symptoms that include tremors and impaired balance, according to the National Institute of Neurological Disorders and Stroke. Slowness of movement is the classic feature, said Dr. Vikas Kotagal, an associate professor of neurology at the University of Michigan.

As symptoms progress, people with Parkinson’s disease may struggle to walk, speak and carry out other basic tasks. An estimated three-quarters of people living with Parkinson’s for more than 10 years will develop dementia.

Genetic mutations are linked to about 10 to 15 percent of Parkinson’s diagnoses, said Dr. Michael S. Okun, a neurologist at the University of Florida.