The number of microorganisms that inhabit human body- normal human flora- is estimated to be 38 trillion and equal to the number of human cells. Historically , the human flora have been viewed as friendly immigrant microbes that enhance immunity against pathogens and in general promote better health for humans. This reviewinvestigates the origin of human flora for it may have important implications to combat opportunistic infections. Of importance, the breast milk and tissue bacteria have been demonstrated not to be contaminants from skin. The unique composition of the bacterial communities in breast milk being different from bacteria from other body parts ,makes it less likely that they were translocated from gut or oral cavity, suggesting either bacteremia or transformation from organic matter as the possible pathways of origin. The documented endogenous origin of the bacteria in breast milk and tissue, the presence of microorganisms in several anatomically well insulated human organs along with the data consistent with the possible endogenous origin of Malassezia and H.Pylori , two species of the normal human flora. suggest at least some of the 38 trillion bacteria inhabiting the human body may not be foreign immigrants .The origin of normal human floraand the precise mechanisms and pathways of origin remains unknown yet the evidence is consistent with an endogenous origin., Further experimental validation of these observations are necessary. A more complete understanding of the human flora may be of help for intelligentstrategies for preventing, diagnosing and treating opportunistic infections.

human flora, bacteremia, endogenous infections, malassezia, h.pylori, breast milk bacteria