P. L. Simond1 summarizes a careful research concerning the mode of spreading of bubonic pest in the following important conclusions: The study shows that the pest is spread by two principal factors—the rat and man. Man is the ordinary agent of transport for long distances, which could not be attained by the rat; the rat is the carrier from house to house. It is much more important in this respect than man, and plays the essential rôle in the dissemination of the disease in local epidemics.
When pestiferous rats enter a new district there is usually some delay before the disease appears in epidemic form; the arrival of infected human patients into a new territory is not always followed by epidemics of pest. In order that epidemics may be established there must be a circle of favorable circumstances, among which the preliminary transmission of the virus to rats seems