Lyme disease is caused by the bacterium Borrelia burgdorferi and rarely, Borrelia mayonii. It is transmitted to humans through the bite of infected black-legged ticks. It is the most common vector-borne disease in the United States.It is transmitted to humans through the bite of infected blacklegged ticks. Typical symptoms include fever, headache, fatigue, and a characteristic skin rash called erythema migraines. If left untreated, infection can spread to joints, the heart, and the nervous system.