lyme disease is caused by infections of the bacterium borrelia burgdorferi which is transferred to humans via blood-sucking bites from the tick, ixodes scapularis. newly hatched ticks do not harbor b. burgdorferi. rather, they pick it up from certain hosts that are reservoirs of the bacterium. when a larval tick crawls onto a host, it may get groomed off and killed by the host, or it may feed successfully, in which case it may or may not then become infected with b. burgdorferi.