How Do Animals Such as Bacteria Attain 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 sūtras; likewise, how harmful bacteria became harmful bacteria cannot be traced to their origin. However, harmful bacteria are only harmful to some sentient beings, and are not necessarily harmful to all sentient beings throughout the dharmadhātu, nor do they necessarily remain reborn as harmful bacteria forever. Species of life undergo variation within the cycle of birth and death, because although fungal and bacterial beings are extremely ignorant and deluded, they still possess extremely faint mental activity of consciousness and very weak karmic actions. As long as there are karmic actions, 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 realms of rebirth spoken by the Buddha in the Śūraṅgama Sūtra (Laṅkāvatāra Sūtra).

Large carnivorous animals generally were humans in past 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, this is still unknown.