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. A few months after the bite, muscle paralysis , joint inflammation , neurological symptoms and sometimes heart symptoms may occur. The initial infection is called primary Lyme disease .