Multiple Sclerosis is an immune system disease which causes your white blood cells to attack the protective sheath (myelin) that covers nerve fibers. As a result, this causes communication problems between your brain and the rest of your body. Eventually, the disease can cause permanent damage or deterioration of the nerves. Thus, permanently disabling a person from using whatever motor functions those nerves are attached to!
Unfortunately, not much is known about what causes Multiple Sclerosis to occur within the human body. Currently, scientists think it's triggered by several factors that relate towards a person's environment, immune system, and genetics.
Symptoms of Multiple Sclerosis include, and aren't limited to, fatigue, numbness or tingling, walking (gait) difficulties, pain and itching, bowel problems, depression, speech problems, sensitivity to heat or cool temperatures, breathing problems, hearing loss, emotional changes, and so on.