The difference lies in the direction of the damage. If it comes from a cell to mitochondria then it is a normal situation we all (including your doctor) are use

The difference lies in the direction of the damage. If it comes from a cell to mitochondria then it is a normal situation we all (including your doctor) are used to. But if the damage comes from inside the mitochondria to a cell, leading to its dysfunction and eventually death, then we are talking about a so called mitochondrial dysfunction. There is a big difference between these two conditions. The first one is natural and healthy, while the second one is pathological.