Acharya Prashant explains the three layers of human existence: the physical body, the subtle body consisting of thoughts, and the causal body which is the root ego. He describes the physical body as an incomplete product of evolution, a work in progress that is naturally susceptible to defects and diseases. He emphasizes that the body is inherently a home for ailments, and even if one escapes specific illnesses, old age remains an unavoidable condition. He suggests that physical health is a result of collective karma, where the actions of humanity affect the individual, much like a pollutant in the ocean affects all its waves.