Guillain-Barr syndrome is an autoimmune disorder that affects the nerves. Autoimmune disorders occur when the immune system malfunctions and attacks the body's own tissues and organs. In Guillain-Barr syndrome, the immune response damages peripheral nerves, which are the nerves that connect the central nervous system (the brain and spinal cord) to the limbs and organs. Specifically, the immune response affects a particular part of peripheral nerves called axons, which are the extensions of nerve cells (neurons) that transmit nerve impulses. Guillain-Barr syndrome can affect the neurons that control muscle movement (motor neurons); the neurons that transmit sensory signals such as pain, temperature, and touch (sensory neurons); or both. As a result, affected individuals can experience muscle weakness or lose the ability to feel certain sensations.  Muscle weakness or paralysis are the characteristic features of Guillain-Barr syndrome. The weakness often begins in the legs and spreads to the arms, torso, and face and is commonly accompanied by numbness, tingling, or pain. Additional signs and symptoms of the condition include difficulty swallowing and difficulty breathing. Occasionally, the nerves that control involuntary functions of the body such as blood pressure and heart rate are affected, which can lead to fluctuating blood pressure or an abnormal heartbeat (cardiac arrhythmia).  There are several types of Guillain-Barr syndrome, classified by the part of the peripheral nerve involved in the condition. The most common type of Guillain-Barr syndrome is acute inflammatory demyelinating polyradiculoneuropathy (AIDP). In AIDP, the immune response damages myelin, which is the covering that protects axons and promotes the efficient transmission of nerve impulses. In two other types of Guillain-Barr syndrome, acute motor axonal neuropathy (AMAN) and acute motor-sensory axonal neuropathy (AMSAN), the axons themselves are damaged by the immune response. In AMAN, only the axons of motor neurons are damaged. In AMSAN, the axons of sensory neurons are also damaged. Because of sensory nerve damage, affected individuals can lose the ability to sense the position of their limbs and can have abnormal or absent reflexes (areflexia).  Miller Fisher syndrome, another type of Guillain-Barr syndrome, involves cranial nerves, which extend from the brain to various areas of the head and neck. Miller Fisher syndrome is characterized by three features: weakness or paralysis of the muscles that move the eyes (ophthalmoplegia), problems with balance and coordination (ataxia), and areflexia. People with this condition can have other signs and symptoms common in Guillain-Barr syndrome, such as muscle weakness.  Guillain-Barr syndrome occurs in people of all ages. The development of the condition usually follows a pattern. Prior to developing the condition, most people with Guillain-Barr syndrome have a bacterial or viral infection. The first phase of Guillain-Barr syndrome, during which signs and symptoms of the condition worsen, can last up to four weeks, although the peak of the illness is usually reached in one to two weeks. During the second phase, called the plateau, signs and symptoms of Guillain-Barr syndrome stabilize. This phase can last weeks or months. During the recovery phase, symptoms improve. However, some people with Guillain-Barr syndrome never fully recover and can still experience excessive tiredness (fatigue), muscle weakness, or muscle pain.