Borrelia burgdorferi is the bacteria that causes Lyme disease. B. burgdorferi is transmitted by blacklegged ticks that are commonly found on vegetation and animals such deer, dogs, mice, horses, and birds in endemic areas. When a person is bitten by a tick carrying B. burgdorferi, the bacteria can enter the body through the skin, often causing a characteristic rash, fever, headache, and fatigue. Untreated Lyme disease can spread throughout the body, affecting the joints, heart, and nervous system. Generally, Lyme disease is effectively treated with a few weeks of antibiotics.