How Do Bacteria and Other Beings in the Animal Realm Attain Human Rebirth?

How bacteria came to be bacteria cannot be traced back to any concrete examples, evidence, or scriptural basis found in Buddhist texts. Similarly, the origins of harmful bacteria cannot be traced to their source. However, harmful bacteria are only detrimental to a portion of sentient beings; they are not necessarily harmful to all sentient beings in the Dharma realm, nor are they destined to reincarnate as harmful bacteria forever. Life forms undergo transformations within the cycle of birth and death because, despite the profound ignorance of bacterial beings, they still possess extremely faint conscious mental activities and very subtle karmic actions. Wherever there are karmic actions, there are karmic retributions, and life will inevitably undergo changes. As for large animal species, once their karmic retribution concludes, they will gradually reincarnate as humans. For specifics, refer to the truth of the six realms of rebirth as taught by the Buddha in the Śūraṅgama Sūtra.

Large carnivorous animals generally had human existences in previous lives. Having committed evil karma, they were reborn as large animals. Once this evil karma is exhausted, they will naturally return to human form. The human body represents a crucial turning point, capable of leading either upward or downward. As for whether one can be reborn as the most ignorant bacteria, it remains unknown.