Claude Bemard made an important contribution to medical ecology when he estabíished the concept of "milieu intárieur" - as opposed to that of the external environment. This conception should be viewed not only as a major step for the understanding of physiological processes and homeostasis, but also as a starting point for the study of the endogenous ecology of individual organisms. As Shelford and Chapman have pointed out independentiy, at this level, ecology and physiology have much in common. Medical ecology, at the individual level, involves the study of the interrelationships of man and his individual microbiota. Claude Bemard identified the abiotic factors of the internal environment, at a time when Pasteur and Koch where engaged in the identification and description of the biotic components of the internal ecosystem.