How Do Bacteria and Other Beasts Obtain a Human Body?

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, how harmful bacteria became harmful bacteria cannot be traced back to its origin. 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, because although fungal and bacterial beings are extremely ignorant and deluded, they still possess extremely faint activities of the conscious mind and very faint karmic formations. As long as there are karmic formations, there will be karmic retribution, 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 realms 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 from it. As for whether one can be reborn as the most ignorant and deluded bacteria, that is still unknown.