The intestinal system of all animals is home to a complex ecosystem of millions of microorganisms, known as the microbiome, previously termed the microflora. The intestinal microbiome contains a variety of different bacteria and fungi of which there are typical ~ 400 different types of microorganisms with a total population of ~ 1014 throughout the length of the intestinal tract. This means there are more organisms in the gut than there are cells in the rest of the body, and that makes the intestinal system the largest immune organ in the body. The number of microorganisms increases numerically from the stomach through to the large intestine, where the microbial diversity is higher due to the slower flow of food and the increased time and availability of nutrients for the microbiome to utilize.