GRE Reading Comprehension: PPO-GRE阅读PPO - M7761R688H21S2ZBS

The immune systems of hamsters injected with laboratory cultures of Borrelia burgdorferi, the bacterium that causes Lyme disease, respond differently than do the immune systems of people infected with the bacterium as a result of the bites of ticks, the disease's carrier. However, when hamsters are infected with the bacterium by tick bites, their immune-system response is identical to the human one. Probably, therefore, the bacterium in the ticks has a different form from the bacterium cultured in the laboratory.