The above experiments demonstrate that guinea pigs and rats subjected to vitamin-deficient diets to a point at which deficiency symptoms appear, and then inoculated with typhus virus, exhibit clinical pictures which indicate a far more severe infection than that observed in normal animals after inoculation. There is also a wider distribution of Rickettsiae and a concentration of organisms which, in pleural and peritoneal exudates, amounts to almost cultural proportions.

Important from our point of view is the fact that these experiments furnished a step toward the accomplishment of our purpose, which was to obtain amounts and concentrations of Rickettsiae suitable for immunological studies until such a time when tissue culture may have developed to a practically useful stage.

The experiments are of immediate importance in that they furnish us a method for improving our technique of active immunization reported upon in the preceding paper, No. V (8).

From the epidemiological point of view these experiments at least suggest an explanation of one of the important factors which enter into the historical association of high typhus mortality with war and famine.

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