There are many beneficial microorganisms living in our bodies， some of them are already not able to be separated from human bodies. As the Nobel Prize winner Lederberg said： these microorganisms and the human body have consist a super living body. By Xie Yuetian
　　Navigation
　　for the microorganisms
　　In a few hours after human’s birth， microorganisms come into human bodies by breathe， breast nursing and many other ways， they take up suitable locations and begin to reproduce， this is the origin of normal microorganisms for human bodies. However not all the microorganisms could have colonization smoothly. The scientists doubt that for the necessary microorganisms of human bodies （especially the intestinal bacteria）， the body may continually has conversations with them by some chemical signal， and navigate them to the suitable location； if it’s not the truth， it will be difficult to explain how could microorganisms locate so accurately， and why the immune system of human doesn’t resist to them.
　　Cooperation
　　of the microorganisms
　　Obviously the microorganisms understand the win-win principle. Normal microorganisms in human body enjoy the living space while returning for human gratefully. Firstly， they can stop the foreign pathogenic microorganisms. The normal microorganisms of human body could also compound some nutriments necessary for human， the enteric microorganisms are good at this. Many intestinal bacteria could make multivitamin and amino acid for human body to absorb.
　　Destruction
　　by the microorganisms
　　Many oral diseases are not cause by foreign microorganisms， but the change of the oral microorganisms. If the sugar content of food becomes higher， and one pays little attention on the oral hygiene， the bacterial colonies that produce acid will form on the surface of teeth， the acid will dissolve the enamel of the teeth surface. If the solid protection is broken by bacteria， they will resolve the inner organizations of the teeth easily， and results in decayed teeth. Fluorine can improve the teeth’s resistance to corrosion and restrain the sugar metabolism by bacteria， so we put moderate fluoride into the drinking water and toothpaste to prevent dental caries.
　　Balance
　　of the microorganisms
　　As well as the macro ecosystem， the micro ecosystem should also keep in balance. This balance is crucial to human health. For instance， the abuse of antibiotic may result in alteration of intestinal flora， as the antibiotic kills many normal intestinal bacteria and make the harmful microorganisms reproduce. At this time， the most common response of human is ceaseless diarrhea. Scientists found that according to the different dietary habits， the compositions， structures and functions of the intestinal bacteria have big differences. Some research found that there are microbial strains that can secrete algae metabolic enzymes in the Japanese’s intestinal tracts， and these microbial strains are the peculiar microorganisms for the Japanese， the reason may be that the Japanese people have eaten algae as food for a long time， and ultimately the microbial strains that can secrete algae metabolic enzymes have colonization in the intestinal tracts.
　　Diseases
　　caused by microorganisms
　　It should be pointed out that， the normal microorganisms of human body should behave themselves and stay at their places to keep “normal”. Once they stroll to other places in the body for some reasons， they may always lead to hazards and become the chief criminal who cause the diseases. That is the endogenous infection often brought by the doctors. For example， sometimes the escherichia coli go to the prostate and cause bacterial prostatitis..