Valerie Brown has a long, humbling piece on the micro-organisms that make up "the vast majority estimated by many scientists at 90 percent of the cells in what you think of as your body":
Researchers have found several reasons to believe that bacteria affect the mental health of humans. For one thing, bacteria produce some of the same types of neurotransmitters that regulate the function of the human brain. The human intestine contains a network of neurons, and the gut network routinely communicates with the brain. Gut bacteria affect that communication. “The bugs are talking to each other, and they’re talking to their host, and their host talks back,” Young says. The phrase “gut feeling” is probably, literally true.