430 PATHOGENIC BACTERIA. tents, and Kitt, who discovered them in this position, points out that the infection of swine probably takes place by the entrance, along with the food, of the fecal matter of diseased animals into the alimentary apparatus of others. Pasteur, Chamberland, Roux, and others have worked upon a protective vaccination based upon the attenuation of the virulence of the organism by passing it through rabbits. Two vaccinations are said to be necessary to produce immunity. The vaccinated animals, however, may be a source of infection to others, and should always be isolated. Klemperer in 1892 found that the blood- serum of immunized rabbits would save infected mice into which it was injected. Ivorenz in 1894 found an antitoxic substance in the blood of rabbits immunized to the disease. The effect of its injection into other animals is, however, only a temporary immunity. Later1 he found it possible to protect hogs against the disease by injecting them first with a serum obtained from a hog immunized in the ordinary manner described by Pasteur, afterward with a feeble culture of the bacillus, and finally with viru- lent cultures. The strength of the serum should be determined by injecting varying quantities of it into mice infected with definite amounts of a culture of known virulence. The immunity prodticed by Lorenz lasted for a year. 1 CentralbLf. Bakt. u. Parasitenk.^ Jan., 1896, p. 168.