A diverse milieu of harmless microbes thrives on the surface of the human body. These human-associated microbes comprise an enormous collection of prokaryotes (archaea and bacteria), eukaryotes (fungi and protozoa), and viruses. The discovery of universal phylogenetic taxonomic molecular markers and the availability of robust deoxyribonucleic acid (DNA) sequencing tools have enabled the identification of several previously unknown human-associated microbes. Consequently, the Human Microbiome Project (HMP)---the first comprehensive survey of the human-associated microbes---has determined the microbial diversity and its abundance in health and disease. HMP study shows that human adults have a similar microbial composition as that of higher taxonomic level (phylum), although uniquely differing from lower taxonomic level (genus and species). Bacteria are the predominant microbial constituent of the human body, and the large intestine (the lower gut), especially, is the most densely populated microbial niche. The human gut is estimated to have over 100 trillion microbes encompassing over 1000 bacterial species, outnumbering the total human body cells by a factor of ten. Gut microbes have a significant impact on human physiology through their role in protection against gut infections, expanding nutrient harvest, educating the infant immune system, modulating drug efficacy, and so forth. The gut microbial communities are collectively recognized as an `organ' for their indispensable contribution to health. Gut microbes supplement human biology with numerous functional genes, metabolic pathways, bioactive metabolites, etc. The perturbation of gut microbiota composition has a pathological impact on human physiology. Hence, the ensemble of the microbial genetic material associated with us represents `our acquired genome'. Overall, the human-microbial synergistic interaction is an evolutionary amalgamation of three domains of cellular life---Archaea, Bacteria, and Eukarya---mutually co-existing with acellular viruses, and collectively referred to as a `superorganism'.
