Each human harbors a teeming ecosystem of microbes that outnumbers the total number of cells in the human body by a factor of 10 to one and whose collective genome is at least 150 times larger than their own. In 2012 the NIH completed the first phase of the Human Microbiome Project, a multi million-dollar effort to catalogue and understand the microbes that inhabit human bodies. The microbiome varies dramatically from one individual to the next and can change quickly over time in a single individual. Here, Velasquez-Manoff discusses how, amid the trillions of microbes that live in the intestines, scientists have found a few species that seem to play a key role in keeping humans healthy.