Borrelia burgdorferi transmitted by the bite of a deer tick .
Most cases occur in the Northeast, upper Midwest, and the Pacific Coast.
Mice and deer are the most commonly infected animals that serve as host to the tick . Most infections occur in the summer. The disease is difficult to diagnose because the symptoms mimic other diseases. In secondary Lyme disease, the infection spreads via the lymph or blood within days to weeks after the tick bite . Symptoms may involve the skin, the central nervous system , and the musculoskeletal system, and they may be intermittent and usually disappear after days or months.