The course is considered of a second type of Salmonella infection naturally appearing in a guinea pig population during the endemic stage of an earlier outbreak. After a quiescent period of 5 months the percentage mortality increased abruptly; fluctuated, with a second rise during the 9th month; and then declined. With the exception of a high rate during the 2nd month the percentage mortality from the initial infection tended to remain on a low level.
The spread of infection in the cages of the breeding stock is recorded from the time of the first fatal case. There was a slow but general dissemination of the second organism through the group. Fatal cases were confirmed solely to the sows. It is suggested that a lowered individual resistance occuring during pregnancy might be associated with the regular cage spread and with the apparent difference in susceptibility of the sexes.
Natural host resistance, virulence of the organisms and acquired host resistance are discussed from the standpoint of their bearing on the unequal distribution of deaths from the two infections.