Motor Neurone disease, also known as Amyotrophic lateral sclerosis (ALS) and Lou Gehrig's disease, are rare, progressive, neurological conditions that involve degeneration of the nerve cells (neurons) responsible for controlling voluntary muscle movement.
“In ALS, both the upper motor neurons and the lower motor neurons degenerate or die, and stop sending messages to the muscles. Unable to function, the muscles gradually weaken, start to twitch (called fasciculations), and waste away (atrophy). Eventually, the brain loses its ability to initiate and control voluntary movements.”
“The cause of ALS is not known, and scientists do not yet know why ALS strikes some people and not others. However, evidence from scientific studies suggests that both genetics and environment play a role in the development of ALS.”
Source: US National Institute of Neurological Disorders and Stroke (NINDS)
Although in the majority of cases, the cause is not known, it's estimated that about 5–10% of cases are inherited.
Many other factors about the disease(s) are yet to be discovered :
1) How the disease starts / progresses
2) The very large variations in survival times
3) Any possible reversal treatments