Pasteur and the bacteriologists of his time discovered that bacteria cease to grow in artificial culture media after a time, because of the exhaustion of the food material in some cases and because of the injurious action of their own products in other instances. These facts were brought forward to explain immunity shortly after bacteria were shown to be the cause of certain diseases. Theories based on these observations were called (1) “_Exhaustion Theory_” of _Pasteur_, and (2) “_Noxious Retention Theory_” of _Chauveau_ respectively. The fact, soon discovered, that virulent pathogenic bacteria are not uncommonly present in perfectly healthy animals, and the later discovery that immunity may be conferred by the injection of dead bacteria have led to the abandonment of both these older ideas. The (3) “_Unfavorable Environment_” theory of _Baumgartner_, _i.e._, bacteria do not grow in the body and produce disease because their surroundings are not suitable, in a sense covers the whole ground, though it is not true as to the first part, as was pointed out above, and is of no value as a working basis, since it offers no explanation as to _what the factors are_ that constitute the “_unfavorable environment_.” Metchnikoff brought forward a rational explanation of immunity with his (4) “_Cellular or Phagocytosis Theory_.” As first propounded it based immunity on the observed fact that certain white blood corpuscles, _phagocytes_, engulf and destroy bacteria. Metchnikoff has since elaborated the original theory to explain facts of later discovery. Ehrlich soon after published his (5) “_Chemical or Side-chain Theory_” which seeks to explain immunity on the basis of _chemical substances_ in the body which may in part destroy pathogenic organisms or in part neutralize their products; or in some instances there may be an absence of certain chemical substances in the body cells so that bacteria or their products _cannot unite_ with the cells and hence can do no damage. [Illustration: PLATE VI PAUL EHRLICH] At the present time it is generally accepted, in this country at least, that Ehrlich’s theory explains immunity in many diseases as well as many of the phenomena related to immunity, and in other diseases the phagocytes, frequently assisted by chemical substances, are the chief factors. Specific instances are discussed in _Pathogenic Bacteriologies_ which should be consulted. It is essential that the student should be familiar with the basic ideas of the chemical theory, not only from the standpoint of immunity, but also in order to understand the principles of a number of valuable methods of diagnosis.