The spontaneous appearance of a second paratyphoid infection in a guinea pig population during the endemic stage of an earlier epidemic is reported. A comparative study of the gross pathology of the two infections indicated a difference in the loci of development of the respective organisms in the animal host. The two types were readily differentiated by direct agglutination with specific immune serums. From its agglutinative affinities the second organism was judged to be an aertrycke type of B. paratyphi.

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