Lyme disease mimics many different diseases. The EM lesion may be confused with streptococcal cellulitis, erythema multiforme (the latter lesions tend to be smaller, urticarial, or vesicular and may occur on mucosal surfaces), and erythema marginatum (these lesions are smaller and migrate rapidly in minutes to hours). Lyme arthritis can be distinguished from other rheumatoid diseases, such as acute rheumatic fever, based on the EM lesion and the brief episode of synovitis. The chronic form of Lyme arthritis may resemble pauciarticular juvenile rheumatoid arthritis, psoriatic arthritis, Reiter’s syndrome, and reactive arthritis caused by members of the Salmonella, Shigella, Campylobacter, and Yersinia genera. This form of arthritis may also be associated with rubella, hepatitis B, or echoviruses. The aseptic meningitis in Lyme disease may resemble enteroviral, leptospiral, or early tuberculous meningitis. It is important to consider sarcoidosis, Behรงet’s disease, and multiple sclerosis when the disease becomes chronic.