Lyme Disease Lyme Disease
National Institute of Allergy and Infectious Diseases

Lyme Disease Introduction

Lyme disease (borreliosis) is the most prevalent tickborne infectious disease in the United States. The disease is caused by a spiral-shaped bacterium, Borrelia burgdorferi, and transmitted to humans by the black-legged tick, Ixodes scapularis.


Typically, the first symptom of Lyme disease is a red rash known as erythema migrans (EM). The telltale rash starts as a small red spot at the site of the tick bite and expands over time, forming a circular or oval-shaped rash. As infection spreads, rashes can appear at different sites on the body. Erythema migrans is often accompanied by symptoms such as fever, headache, stiff neck, body aches, and fatigue.

After several months of B. Burgdorferi infection, slightly more than half of people not treated with antibiotics develop recurrent attacks of painful and swollen joints, most commonly in the knees. About 10 to 20 percent of untreated people develop chronic arthritis.

Lyme disease can also affect the nervous system, causing such symptoms as stiff neck, Bell’s palsy, and numbness in the limbs. Less commonly, untreated people can develop heart problems, hepatitis, and severe fatigue.


Healthcare providers may have difficulty diagnosing Lyme disease because many of its more common symptoms are similar to those of other disorders and viral infections. In addition, the only distinctive sign unique to Lyme disease—the EM rash—is absent in at least one-fourth of the people who become infected