What is the key symptom of Lyme disease?

The key symptom of Lyme disease is erythema migrans, which is a characteristic rash that often appears at the site of a tick bite. This rash typically presents as a circular, expanding erythematous lesion with a central clearing, resembling a "bull's-eye." The appearance of erythema migrans is significant because it not only helps in diagnosing Lyme disease but is also a strong indicator of the presence of the infection.

While fever and chills, cough and congestion, and joint swelling can occur as symptoms of Lyme disease or in other illnesses, they are not exclusive or definitive indicators of the disease. For instance, fever and chills may be associated with many types of infections, and cough and congestion are more commonly linked to respiratory infections. Joint swelling can occur later in the disease's course but is less specific as a primary symptom. In contrast, erythema migrans specifically helps identify Lyme disease early, allowing for timely treatment and preventing potential complications.