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1. Different strains of Trypanosoma lewisi represent different states of biological balance, especially between the powers of propagation and resistance to destruction. 2. The biological status of a given strain of Trypanosoma lewisi is subject to cyclic variations as the result of immunological reactions in the blood of the host. 3. The factors limiting reproduction and causing destruction of Trypanosoma lewisi in the blood are appreciably independent of each other. It is possible, therefore, to influence these processes separately and even in opposite directions. 4. The virulence of Trypanosoma lewisi, manifested in its highest form, is dependent upon some degree of reproductive fastness, strong antigenic action, and susceptibility to destruction, varying degrees in the development of these properties producing corresponding variations in the degree of virulence. 5. By a properly regulated system of passage the properties of Trypanosoma lewisi that determine its infection cycle and its virulence may be eventually so altered as to change completely both the nature and course of the infection. Such a system of passage must be adapted to the particular strain of Trypanosoma lewisi used. 6. Immunological reactions exercise a dominant influence in determining the ultimate biological variations of Trypanosoma lewisi.

CONCERNING CHANGES IN THE BIOLOGICAL PROPERTIES OF TRYPANOSOMA LEWISI PRODUCED BY EXPERIMENTAL MEANS, WITH ESPECIAL REFERENCE TO VIRULENCE