Lyme disease, also known as Lyme borreliosis, is a vector-borne disease caused by the Borrelia bacterium, which is spread by ticks in the genus Ixodes. The most common sign of infection is an expanding red rash, known as erythema migrans (EM), which appears at the site of the tick bite about a week afterwards. The rash is typically neither itchy nor painful. Approximately 70–80% of infected people develop a rash. Early diagnosis can be difficult. Other early symptoms may include fever, headaches and tiredness. If untreated, symptoms may include loss of the ability to move one or both sides of the face, joint pains, severe headaches with neck stiffness or heart palpitations. Months to years later repeated episodes of joint pain and swelling may occur. Occasionally shooting pains or tingling in the arms and legs may develop. Despite appropriate treatment, about 10 to 20% of those affected develop joint pains, memory problems and tiredness for at least six months.

Lyme disease is transmitted to humans by the bites of infected ticks of the genus Ixodes. In the United States ticks of concern are usually of the Ixodes scapularis type and must be attached for at least 36 hours before the bacteria can spread. In Europe Ixodes ricinus ticks may spread the bacteria more quickly. In North America the bacteria Borrelia burgdorferi and B. mayonii cause Lyme disease. In Europe and Asia Borrelia afzelii, Borrelia garinii, B. spielmanii and four other species also cause the disease. The disease does not appear to be transmissible between people, by other animals nor through food. Diagnosis is based on a combination of symptoms, history of tick exposure and possibly testing for specific antibodies in the blood. Blood tests are often falsely negative in the early stages of the disease. Testing of individual ticks is not typically useful.

Prevention includes efforts to prevent tick bites by wearing clothing to cover the arms and legs and using DEET or picaridin-based insect repellents. Using pesticides to reduce tick numbers may also be effective. Ticks can be removed using tweezers. If the removed tick is full of blood a single dose of doxycycline may be used to prevent the development of infection but is not generally recommended since the development of infection is rare. If an infection develops, a number of antibiotics are effective, including doxycycline, amoxicillin and cefuroxime. Standard treatment usually lasts for two or three weeks. Some people develop a fever and muscle and joint pains from treatment, which may last for one or two days. In those who develop persistent symptoms, long-term antibiotic therapy has not been found to be useful.

Lyme disease is the most common disease spread by ticks in the Northern Hemisphere. Infections are most common in the spring and early summer. Lyme disease was diagnosed as a separate condition for the first time in 1975 in Lyme, Connecticut. It was originally mistaken for juvenile rheumatoid arthritis. The bacterium involved was first described in 1981 by Willy Burgdorfer. Chronic symptoms following treatment are known as "post-treatment Lyme disease syndrome" (PTLDS). PTLDS is different from chronic Lyme disease, a term no longer supported by scientists and used in different ways by different groups. Some healthcare providers claim that PTLDS is caused by persistent infection but this is not believed to be true because no evidence of persistent infection can be found after standard treatment.

No human vaccines for Lyme disease are currently available, although research is ongoing. Multiple vaccines are available for the prevention of Lyme disease in dogs.