Lyme disease is an illness caused by bacteria (usually Borrelia burgdorferi). The bacteria that transmits Lyme disease is commonly found in the black-legged deer tick. This tick transmits many human diseases, so infection with other diseases along with Lyme is possible. Current science suggests that Lyme disease is transmitted only after many hours of the tick being attached, so removing the tick promptly can dramatically reduce the risk of transmission.
Lyme disease usually starts with a rash (often a very specific bullseye-type rash called erythema migrans), fever, muscle aches, and swollen lymph nodes. Because these symptoms are common with lots of illnesses, it’s important to tell your doctor about known tick bites, or if you think you might have been exposed to ticks (like if you’ve been hiking or clearing brush). Recognizing Lyme disease early leads to better outcomes. Your doctor will base diagnosis on your signs and symptoms, tick exposure history, and possibly lab tests. Lab tests for Lyme disease are not recommended if you do not have symptoms because there may be false positives.
Untreated Lyme disease can lead to arthritis, nerve damage, heart problems (Lyme carditis), and other serious symptoms. Even after treatment, some people experience muscle and joint pain for several months (post-Lyme disease treatment syndrome). Lyme disease is not chronic and with treatment full recovery is expected.
The CDC has many more helpful resources for understanding Lyme Disease.