EXPERIMENTAL EPIDEMIOLOGY OF TUBERCULOSIS : HEREDITARY RESISTANCE TO ATTACK BY TUBERCULOSIS AND TO THE ENSUING DISEASE AND THE EFFECT OF THE CONCENTRATION OF TUBERCLE BACILLI UPON THESE TWO PHASES OF RESISTANCE.

1. Hereditary resistance to attack by air-borne tubercle bacilli is distinct from the resistance to the ensuing disease. 2. One inbred rabbit family has little resistance to attack: by the microorganism but has considerable resistance against the ensuing disease. 3. Another inbred family has considerable resistance against attack by the tubercle bacillus but has little resistance against the ensuing disease. 4. Increasing concentrations of tubercle bacilli in the environment of the family of high genetic resistance to the disease increase the incidence of infection, accelerate the onset of the disease, and affect its essential character in proportion to the concentration of the microorganism. 5. Up to a certain concentration of tubercle bacilli in the environment of the families of low genetic resistance to the disease, increasing concentrations of the infectious agent also increase the incidence of the disease and accelerate its onset, although its anatomical character is always of a uniform rapidly progressive type. Beyond this concentration further increment of the infectious agent exercises no effect.