Myasthenia gravis is an autoimmune disorder that causes muscles to weaken easily. After resting, the muscles usually become strong enough to resume activity for a while. Sometimes the symptoms may seem worse and other times you may have no symptoms. In myasthenia gravis, the immune system attacks the receptors used for muscle contraction. Treatment may involve removing the thymus. Follow the links below to find WebMD's comprehensive coverage about how myasthenia gravis is caused, who gets it, how to treat it, and much more.