EXPERIMENTS ON THE CONVERSION OF TYPHUS STRAINS.

TWO TYPES OF STRAINS OF TYPHUS VIRUS ARE OBSERVED IN MEXICO: first the murine type which is obtained from wild rats in nature, from isolated endemic cases, and from cases during short epidemic outbreaks, and second, the epidemic type of strains which is obtained from long standing serious epidemics. Some of these epidemic strains correspond entirely to strains of historic Old World typhus. Other strains which in their experimental behavior are intermediate between these two types of strains were isolated from the same epidemic.