Humans are not alone. They are inhabited by nearly 100 trillion living species of friendly bacteria. The human body is far from being silent and barren. It’s really a community, a planet of its own, teaming with diverse life. The various bacteria living in the body are the central intelligence of the human immune system. The greatest concentration of bacteria is found in the intestines, or the gut, where at least a pound or two of living beings come together in one giant city.