How Do Harmful Bacteria and Carnivorous Beasts Obtain Human Bodies?

As for how bacteria became bacteria, if one traces back to the origin, there are no examples or evidence, nor can any basis be found in the Buddhist sutras; likewise, it is impossible to trace back to the origin how harmful bacteria became harmful. However, harmful bacteria are only harmful to some sentient beings, not necessarily to all sentient beings throughout the dharmadhātu, nor are they necessarily reborn as harmful bacteria forever. Species of life undergo variation in the cycle of birth and death (saṃsāra), because although bacterial sentient beings are extremely ignorant and deluded, they still have extremely faint mental consciousness activity and very faint karmic formations (saṃskāra). As long as there are karmic formations, there are karmic retributions, and life will undergo variation. As for large animals, when their karmic retribution ends, they will gradually be reborn as humans; for specifics, refer to the truth of the six paths of rebirth spoken by the Buddha in the Śūraṅgama Sūtra (Lengyan Jing).

Large carnivorous animals generally were humans in previous lives who committed evil karma and were thus reborn as large animals. Once the evil karma is extinguished, they will of course become human again. The human body is an important turning point—one can go up or down. As for whether one can be reborn as the most ignorant and deluded bacteria, this is still not known.