According to this theory, we evolved alongside these microorganisms, and they are vital companions to the human immune system and brain. Just as flowers and bees coevolved and rely on each other, we humans need these bacteria to flourish. In scientific papers published in microbiology and medical journals, the lack of exposure to dirt in modern society is described as “the loss of old friends”—a loss that is linked in humans to an increased risk of mental suffering, including depression.
