Lyme disease is an infectious disease caused by bacteria transmitted through the bite of infected ticks. It is the most common tick-borne illness in the Northern Hemisphere. The disease is contracted when an infected deer tick bites a person and transmits the Borrelia bacteria. Early symptoms are usually flu-like, but later a characteristic bullseye rash may appear. If left untreated, the infection can spread and cause joint, heart, and neurological problems. Treatment involves oral antibiotics, and prevention focuses on tick checks and protective clothing when in wooded areas.