Years of research into Parkinson’s has not discovered what causes nerve cells to die in a part of the brain called the substantia nigra. Ageing is a prime factor but a combination of genes and pollutants or pesticides is believed to be the trigger.
My grandfather died a long, lingering death from Parkinson’s, imprisoned in a crippled body and unable to communicate but with an apparently alert and healthy mind. And pesticides could have been a major factor? Yow.