Any circumstance that provides the physicochemical conditions and nutritional sources suitable for the replication of microorganisms provides their ecological niche for growth. Multicellular organisms, including man and other mammals, also occupy their niche in the natural environment and, in turn, they provide on their surfaces living space for many microorganisms, so that all hosts have a natural microbial flora. It is only by using very artificial techniques that it is possible to produce and maintain “germ-free animals” which have been very valuable in medical research, e.g., to study the roles of normal flora.