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. The number in each cell, in general, depends on how much energy the cell needs.

Mitochondrial diseases are caused by mitochondria not working properly. When mitochondria are not working well, 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.

There are many types of mitochondrial diseases. Some are mild and cause only a few symptoms. Others are serious and can affect many parts of the body. Many mitochondrial disorders are progressive, meaning that symptoms tend to get worse with time.