The writer examines research that correlates microbial populations and human health by comparing the fecal ecosystems found in different individuals. Gut microbes are linked to obesity, colitis, diabetes, and heart disease. Eukaryotic resident-parasites manipulate host behavior, canceling notions of autonomy of the individual. Advances in genomic analyses are driving the study of whole bacterial communities and sequencing of all their genomes. This diversity of organisms is enabling appreciation of the latest topics in the digestive system in genomics, immunology, internal ecosystems, and microbiology. Almost 130 species and over 1,000 bacterial species inhabit the human stomach, more than half of which were previously unknown. The intestines possess an even richer microbial flora. There are 10 times more microbial cells than human cells in the body. Endogenous retroviruses take up four times more space in the human genome than all coding genes. Cheaper and easier sequencing will mean proliferation of data.