The human body has trillions of microbes inhabiting every surface exposed to the environment, both inside and out, termed the microbiome (micro = small, biom = community in a distinct environment). The largest community of microbes is found within the gastrointestinal tract where the gut microbiome in healthy adults is estimated to harbor 1014 bacterial cells, which roughly equates to 100 to 300 times the number of genes in the human genome. This collection of microbes assembles largely through chance, but the assembly is not without design. Microbes have existed on earth for 3.5 billion years, based on the oldest discovered fossilized evidence; humanity has therefore coevolved with its microbial flora, and they play a pivotal role in the neonate, facilitating maturation of the immune system, protection from potential pathogens, and digestion of gut contents for the synthesis of essential vitamins and nutrients.