In 1884 Robert Koch and Friedrich Loeffler published Koch’s Postulates defining our historical understanding of the relationship between an organism and infection: one organism: one disease. In the last decade with research on the microbial community living on and in humans, a new concept of microbial diseases has emerged; that is, alterations of the microbial community can lead to disease including an extension beyond traditional “infectious” diseases to include metabolic disorders such as type 2 diabetes and obesity. As we continue to gain knowledge about the functions of the normal microbiome and the effects of alterations of the microbial population on disease pathogenesis, a new era of diagnostics and therapeutics will evolve. (Ann Clin Microbiol 2013;16: 162-167)