Evidence was presented substantiating the idea that our active agent is a virus. The reactions produced in laboratory animals by the active agent were described, and a comparison of it with other viruses was made. The results of experiments indicating the immunological identity of our virus, Armstrong and Lillie's virus of lymphocytic choriomeningitis, and Traub's virus were given in detail. Finally, the evidence in regard to its natural host and its importance as a factor in diseases of human beings was presented and discussed.

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