Amyotrophic Lateral Sclerosis (ALS), often called Lou Gehrig’s disease, is a rapidly progressive, invariably fatal neurological disease that attacks the neurons responsible for controlling voluntary muscle movement. The disease belongs to a group of disorders, known as motor neuron diseases, which are characterized by the gradual degeneration and death of motor neurons (neurons which activate muscle cells). In ALS, both the upper motor neurons and the lower motor neurons degenerate or die, ceasing to send messages to muscles. The brain is no longer able to initiate and control muscle movement. Unable to function, the muscles gradually weaken, waste away (atrophy), and twitch (fasciculations). Respiratory problems are the most common complications of ALS, and can often lead to infections and pneumonia. Breathing tends to become more difficult as the disease progresses, increasing the risk of respiratory failure.

The cause of ALS is not known, and researchers do not yet understand why ALS strikes some people and not others. Currently, there is no cure for ALS.