Current research is redefining what we think of as a typical animal or human. An animal is really one large organism and a lot of little ones that include viruses, bacteria, and protists. And evolutionary speaking, they are inseparable : hosts and their microbes (and their genes) are intertwined over long periods of evolutionary history. These types of coevolutionary patterns aren't surprising given the symbiotic nature of their relationship. For example, animals provide stable nutrient-rich habitats for microbes, while microbes provide a variety of functions for the animal, affecting the host's physiology, nutrition, immune function, reproduction, and even behavior. Yet, the specific assemblages of microbes we see are extremely diverse across hosts and can be dynamic over time in a given host.