What is myasthenia gravis (MG)? Myasthenia gravis (meye-uhss-THEEN-ee-uh GRAV uhss) (MG) is an autoimmune disease that weakens the muscles. The name comes from Greek and Latin words meaning grave muscle weakness. But most cases of MG are not as grave as the name implies. In fact, most people with MG can expect to live normal lives.