For three years from 1950, the author had made studies on each case of the so-called “Izumi-Fever” which had been prevalent in Okayama prefecture, and had also statistically contemplated the reaults. The author, moreover, made many experiments about the infection routes, using the agent isolated from the patients. The results are as follows: 1) The incidence takes at first the type of mass outbreak and then gradually shifts to that of sporadic one. 2) As for the infection routes, both of the water-borne and droplet infections seem to be possible. 3) There are not rare, but rather many cases of the secondary infection. 4) In order to corroborate the inference obtained by the epidemiological studies on the infection routes, the author gave the pathogenic agent to hamsters and mice by various routes; in this experiment, the alimentary and droplet infections were established, but the one by insects was not successful.