Neurodegeneration
Humanity has come a long way towards understanding life and the universe. However, neurodegeneration is still one of the unexplained mysteries of the brain that scientists are working to solve. The specific type of nerve cell deterioration found in conditions like Parkinson's and Alzheimer's is referred to as neurodegeneration. Though you are aware of hundreds of neurodegenerative disorders, only a few of them, including Alzheimer's, Parkinson's, and Huntington's, have ever undergone research.
What specifically causes neurodegenerative disorders remains a mystery. The only need appears to be advanced age, but that is all. Patients with neurodegenerative illnesses do not appear to be related in any way, other than by age. That makes it very impossible to forecast and, thus, properly treat some of the most severe mental diseases that medicine is aware of. You also don't know exactly how these illnesses assault and kill neurons over time, or what occurs in the brain once they start and continue to progress.