Lyme disease is a bacterial infection transmitted to humans by ticks. People who hike, camp, work, or live in or near wooded areas during summer months are most at risk for Lyme disease. People bitten by an infected tick may develop a large area (diameter >2 in) of redness (rash) around the bite days to weeks later. As this rash gradually expands, a bright red outer border and a central pale area of clearing are often seen (the rash may also be uniformly red). This type of rash is called erythema migrans. Symptoms such as chills, fever, fatigue, headache, and joint pains may accompany the rash or may develop days after the rash appears. If the infection is not treated, symptoms may persist for several weeks. Also, complications involving the heart, joints, or nervous system may develop, sometimes as long as 1 to 2 years later. Doctors often give patients with erythema migrans and early, uncomplicated Lyme disease antibiotics for 3 to 4 weeks. It is not clear whether such lengthy treatment is needed.