If disease is defined as any "deviation from the healthy or normal condition of any of the functions or tissues of the body," it will include both those that are infectious, where the disturbance is due to an invading organism, and the non-infectious, where the disturbance is due directly to some inherited or environmental cause. Inherited non-infectious disturbances, may include a great range of conditions, from those which are definitely lethal, to defectives of various grades, down to those which may have very little vital value and then possibly only under certain conditions. The incidence of infectious diseases depends first on exposure to the infection and second on susceptibility. The susceptibility may be due to environmental or hereditary predisposition. For the study of the latter special precautions must be taken, since in addition to the confusing conditions of the environment, genetic variability of virulence in the infective organism as well as in the susceptibility of the host must be taken into account. A study of the inheritance of susceptibility and resistance of rabbits to Brucella abortus is used as illustrative. Whether it is more practicable from an economic standpoint to establish disease-resistant strains or to control disease through other measures is a matter which must be worked out in each particular case.