Amyotrophic lateral sclerosis (ALS or Lou Gehrig's disease) is a progressive, always fatal neurological disease. It attacks nerve cells  responsible for the control of voluntary muscles in the body. Amyotrophic lateral sclerosis is part of a group of disorders known as "motor neuron diseases," characterized by the slow degeneration and death of motor neurons.

Nerve cells located in the brain, brainstem, and spinal cord, called motor neurons, serve as the controlling units and provide important  communication links between the body's nervous system and the voluntary muscles. Signals transmitted from motor neurons in the brain  are sent to motor neurons in the spinal cord and from them to particular muscles in the body. In amyotrophic lateral sclerosis, both the upper motor neurons and the lower motor neurons degenerate or die, and then cease sending signals to muscles. The muscles slowly become weak, waste away, and twitch. Finally, the brain loses the ability to control any type of voluntary movement.

Amyotrophic lateral sclerosis causes weakness and, eventually, all muscles under voluntary control become affected. Patients with ALS lose strength and the ability to move their legs, arms, and body. Eventually, when the muscles in the diaphragm and chest wall become affected, ALS patients are unable to breathe effectively without the assistance of a ventilator. Unfortunately, most people suffering from ALS succomb to respiratory failure, often within three to five years from the onset of initial symptoms. Ten percent of patients, however, survive ten years or longer.

Amyotrophic lateral sclerosis affects only motor neurons. Because of this, it does not impair a person's mind, personality, or memory in any way. ALS also does not affect sight, taste, smell, hearing, or sensation. ALS patients can usually control their eye muscles and have unaffected bladder and bowel functions.