In about 10% of cases, ALS is caused by a genetic defect. In other cases, the cause is unknown.
In ALS, nerve cells (neurons) waste away or die, and can no longer send messages to muscles. This eventually leads to muscle weakening, twitching, and an inability to move the arms, legs, and body. The condition slowly gets worse. When the muscles in the chest area stop working, it becomes hard or impossible to breathe on one's own.
ALS affects approximately 1 out of every 100,000 people.
Except for having a family member who has a hereditary form of the disease, there are no known risk factors.