Yersinia pestis is extraordinarily virulent, even when compared with closely related bacteria. This is because it's a mutant variety, handicapped both by not being able to survive outside the animals it infects and by an inability to penetrate and hide in its host's body cells. To compensate, Y. pestis needs strength in numbers and the ability to disable its victim's immune system. It does this by injecting toxins into defense cells such as macrophages that are tasked with detecting bacterial infections. Once these cells are knocked out, the bacteria can multiply unhindered.