A seizure occurs when groups of neurons in the brain fire rapidly and uncontrollably, causing abnormal sensations, feelings, movements, or behavior. Epilepsy, a diagnosis applied after repeated seizures, affects about two million people in the United States. It has many possible causes, including tumors, infections, head injury, stroke, poisons, drug overdoses, and other neurological disorders. Often it results from genetically influenced abnormal brain development. In most cases, though, the cause is unknown. Sleep loss, alcohol, infections, and possibly stress may provoke individual seizures in a person who is vulnerable for any reason.