It is a truth of biology that if it can go right it can go wrong.
Billions of things go right in your body at every instant of your life. It is not surprising, given the enormous number of functions at the molecular, cellular, tissue and organ levels needed to sustain life, that things go wrong sometimes. The severity of that “wrong” depends on how much it deviates from normal and how foundational the affected function is. The loss of nerve cells in the substantia nigra in Parkinson’s disease is a very severe loss of function. This has a very significant impact because the substantia nigra is foundational for the initiation of conscious movement, which is crucial to navigate our physical environment and the society we live in.