Small Cell in a Huge Ecosystem
A normal ten year old boy dies while playing with his friends by a river. He is sent to a place that is neither heaven nor hell, and is given a chance for reincarnation. Unfortunately, out of the thousands of organisms he could be reincarnated into, he is reincarnated into the smallest and weakest one - a bacteria cell. God, having pity for him, gives him the chance to become something better.