The bite of an infected ticks. The disease, caused by bacteria of the borrelia type, is transmitted to humans through the bite of infected ticks of the Ixodes genus. Two tick species the black legged tick and the western blacklegged tick are known to transmit the bacterium. Most humans are infected from the bites of immature ticks (nymphs) because adult with Borrelia burgdorferi bacterium are larger and more likely to be discovered.