Until within recent years American authorities, bacteriologists and veterinarians alike, have very generally accepted a certain germ, the bacillus of Salmon and Smith, as the specific cause of hog cholera and another somewhat similar germ as the cause of what was supposed to be a distinct but curiously related disease--swine plague. But within a few years workers in the Federal bureau of animal industry have apparently demonstrated that hog cholera is caused by a living germ so small that it passes easily through germ filters which remove all known forms of the bacillus of Salmon and Smith. It may be interesting to note further that this new germ is so small as to be invisible to the highest available powers of the best microscope. That it is a living organism and not a chemical poison may be very easily demonstrated. The curious relations to this disease of the old bacilli of hog cholera and of swine plague are not well understood, but it seems quite possible that they may play some part in the later development of the disease after the disease processes have been started by the invisible germ. While our old theories and supposed information concerning the cause of hog cholera have been very much disturbed by newer work, it is important to remember that hog cholera is now just as much as before to be recognized as a distinctly infectious disease. It is important to remember also that this infection is absolutely necessary, or there can be no cholera no matter how susceptible animals may be. There can be no cholera without this primary and specific cause any more than there can be plants in our wheat fields without the previous presence of mustard seed. Conditions of soil and climate may favor a rank growth of mustard. Conditions of feed and keep may favor the development and spread of hog cholera. They may decrease resistance and increase susceptibility, but cannot originally cause the disease. It is a rather common experience that hogs kept closely housed and fed, especially with such foods as corn, offer less resistance than do other hogs. In our vaccine work we frequently find hogs of this type which die readily under inoculation with blood of low grade virulence. Hogs of hardier type may become slightly sick or not sick at all with inoculation from the same infectious material. Pampered show herds appear especially susceptible to both natural infection and artificial inoculation.