Lyme disease is a multisystem disorder caused by a microorganism (spirochete) called Borrelia burgdorferi that is transmitted by a tick bite. Lyme disease was first observed in Europe and now is recognized on four continents and in at least 20 countries. The disease is named for the community of Lyme, Connecticut, where residents experienced an epidemic of acute arthritis in the summer of 1975. Since then Lyme disease has become the most commonly diagnosed tick-borne illness in the United States.

The clinical picture of Lyme disease can be divided into three stages, not all of which are exhibited in every case. Stage 1 begins with a characteristic expanding skin rash, which develops at the site of a tick bite as a small blister and expands in a circular manner over a number of days. As the rash expands, central clearing often occurs. This skin rash is usually accompanied by non-specific "flu-like" symptoms. Weeks to months after exposure, some patients develop heart or nervous system abnormalities associated with Stage 2 of the disease. The arthritis that represents Stage 3 of Lyme disease appears within several weeks to two years after Stage 1. Lyme disease morbidity is largely related to this arthritis which usually involves the knee. Most cases occur during the summer and early fall. The initial manifestation of Lyme disease may mimic many other disease entities.

Physicians treat patients with Lyme disease with antibiotics; antibiotic usage must always be under the supervision of a physician. Lyme disease patients treated in early stages of the disease, when they only have a rash and flu-like symptoms, usually respond very well to therapy and recover rapidly. Patients who are not treated until later in the disease generally respond satisfactorily as well; however, in a small proportion of patients, symptoms may continue or recur, making additional antibiotic treatment necessary. Permanent damage to joints occurs in a small number of patients whose disease is not successfully treated.