English: This 2007 photograph depicts the pathognomonic erythematous rash in the pattern of a “bull’s-eye”, which manifested at the site of a tick bite on this Maryland woman’s posterior right upper arm, who’d subsequently contracted Lyme disease.

Lyme disease patients who are diagnosed early, and receive proper antibiotic treatment, usually recover rapidly and completely. A key component of early diagnosis is recognition of the characteristic Lyme disease rash called erythema migrans. This rash often manifests itself in a “bull's-eye” appearance, and is observed in about 80% of Lyme disease patients. (added 2017 : "but similar symptoms are observed in other illnesses not caused by Borrelia burgdorferi infection, such as southern tick–associated rash illness, which has not yet been tied to a specific pathogen"^[1],^[2])

Lyme disease is caused by the bacterium Borrelia burgdorferi, and is transmitted to humans by the bite of infected blacklegged ticks. Typical symptoms include fever, headache, fatigue, and as illustrated here, the characteristic skin rash called erythema migrans. If left untreated, infection can spread to joints, the heart, and the nervous system. Note that there are a number of PHIL images related to this disease and its vectors.