What type of pathogen causes Lyme disease?

Lyme disease is an infection that is spread by the bite of ticks infected with the bacterium Borrelia burgdorferi. Symptoms include fatigue, headache, stiff neck, fever, muscle or joint pain and swelling, and sometimes an expanding red rash. If a rash does develop, it may look like a target or "bull's-eye" in some people. Lyme disease can be difficult to diagnose because its symptoms are similar to many other conditions, and tests do not always detect the bacteria. Lyme disease is usually effectively treated with a short course of antibiotics. If not treated promptly, however, it can lead to complications involving the heart, nervous system, joints, and skin within weeks, months, or even years later.