Lyme disease is a tick-transmitted disease caused by the bacterium Borrelia burgdorferi. It is the most common tick-borne illness in North America and Europe, and is spread through the bite of an infected blacklegged tick. Symptoms can include fever, headache, fatigue, and a characteristic skin rash called erythema migrans. If left untreated, infection can spread to joints, the heart, and the nervous system.