B. influenzae can be found in every case of true clinical influenza. To isolate this organism, which is most abundant in the earlier stages of the disease, it is necessary to exercise care in obtaining a suitable specimen, and since growth requirements of this organism are quite rigid, special selective culture media, such as those suggested by Avery (24) and also by Fleming (25), carefully prepared and adjusted to reaction are essential for successful work.

The various strains of B. influenzae apparently do not differ in kind. This is indicated by the cross agglutination, absorption, and protection tests with strains isolated at different localities during the recent pandemic as well as with those from the epidemic of a few years ago—1915 to 1917.

The toxic substances of B. influenzae show a marked action on the bronchorespiratory tract, thereby predisposing these organs to extensive invasion by the organism itself or to secondary infections.

No marked increase in virulence of B. influenzae has been obtained by passages through laboratory animals. This, in the first place, may be due to the relative insusceptibility of these animals to the infection of this organism; secondly, to the probability that the invasive power of the organism is very limited, infection apparently taking place only when the initial toxicity is severe enough to facilitate such invasion.

No bacteremia is produced by B. influenzae in laboratory animals by a dose approximating a minimum lethal dose regardless of the mode of injection chosen. Rabbits receiving intracranial injections will either die of acute toxemia and show no organisms in the blood stream or lungs, or where such infection passes into the sub-acute stage there is apparently a chance for a few of the organisms to get into the blood stream and to be transferred to such organs as the lungs when these have been rendered susceptible by the toxic substances of the organism.

The pathologic lesions in the rabbit, gross and microscopic, in many respects resemble those of influenza in human beings as observed during the past pandemic.

The injection of B. influenzae into the rabbit intravenously results in a rapid and marked decrease in the polymorphonuclear cells.

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