The Guillain-Barré Syndrome is a disease that usually begins quickly, over a few days, and renders a previously healthy individual weak in the legs and arms and in some cases in the breathing muscles and/or muscles used in swallowing. The reason that the disease develops is because the individual's own immune system - the system of white blood cells and other cells that normally fights infections - becomes misdirected and attacks the peripheral nerves. For this reason it is considered one of the autoimmune diseases, a group that includes rheumatoid arthritis, myasathenia gravis, some forms of thyroid disease, and a diverse group of other disorders linked by the fact that the immune system is responsible for the damage. Normally the peripheral nervous system acts as the "wiring" that carries signals from the central nervous system (CNS)- that is, the spinal cord and brain - out to the muscles to produce movement and to the skin to signal touch and other sensations. Thus damage to the peripheral nerves can produce weakness and numbness. With the elimination of poliomyelitis, the most common disorder that can paralyze previously healthy people over a few days is the Guillain-Barré syndrome. The tests that help with diagnosis include electrical tests of the nerves and muscles, laboratory tests of the cerebrospinal fluid obtained by lumbar puncture, and some blood tests. The disease is self-limited, with muscle strength usually reaching a nadir within 2 to 3 weeks, followed by partial or complete recovery taking place over a few weeks to several months.