Our bodies are made up of billions of small units called cells. Mitochondria (my-toe-kon-dree-uh) are tiny structures within cells that make energy for our bodies to use.  Mitochondria make more than 90 percent of the energy we need to live.  There are hundreds to thousands of mitochondria in each cell.

Mitochondrial diseases are caused by mitochondria not functioning properly. When mitochondria are not working properly, the cell does not have enough energy and may be injured or die.  When many cells of the body do not have enough energy, whole organ systems (such as the nervous system or the digestive system) can be affected.