Multiple sclerosis
In healthy people, myelin sheaths help neurons work.  Electric signals in neurons move quickly through long, narrow axons like electricity in a wire. The myelin is like the insulator around the wire that keeps the signal strong by keeping it from moving out of the wire before the end. In people with MS, infiltration of immune cells causes inflammation within the CNS resulting in the loss of the protective insulator, called "demyelination." The progressive loss of the myelin sheath, as well as the loss of myelin-producing cells, impair the ability of the body to regrow its myelin. Without the protective covering, the signals between neurons do not travel well. Because of this, the mind and body cannot work like they normally do.