(_c_) If the assumption of increasing racial immunity does not consist well with all the facts, more perhaps can be said in favour of the unproved hypothesis that a high proportion of the population are from time to time temporarily immunized by small doses of tubercle bacilli; and their resistance to larger doses of infection thereby increased. Experimentally calves inoculated with small doses of tubercle bacilli remain during the next year or two unaffected by much larger doses of tubercle bacilli, unlike calves not submitted to this treatment. Tubercle bacilli are somewhat widely distributed, though they occur chiefly in the immediate environment of careless consumptive patients; and it is conceivable that minimal doses of bacilli may arouse the resistance of the cells and fluids of the body and prepare them to resist successfully larger doses of infection. This is consistent with the fact that while one in about ten deaths from all causes is caused by tuberculosis, a majority of the total population are shown by pathological evidence to have been at one time or another infected by tuberculosis, and yet have either never been ill, or have recovered, usually without the existence of tuberculosis being detected or even suspected. Obviously this is satisfactory evidence that mankind is relatively resistant to the infection of tuberculosis. The fact just mentioned naturally leads to the question: what determines the result when tubercle bacilli invade the human subject? Assuming fairly uniform virulence of tubercle bacilli, the result for an infected person depends on two factors: the dosage of infection, and the resistance of the cells and fluids of the invaded person; and evidently increase in the dosage of infection and lowering of personal resistance may have identical effect in determining serious disease. Of the importance of the already mentioned factors which lower personal resistance to disease,—often also at the same time increasing infection,—there can be no doubt. It is impossible in most instances to set out separately circumstances increasing infection from circumstances lowering resistance. During the last three or four decades there has been improvement in respect of the factors lowering resistance to attack, but there has been simultaneously a great decline in opportunities for infection on a massive scale, as a result of habits of greater cleanliness, especially in regard to spitting, of diminished overcrowding of population, and of increased treatment and the incidental segregation of advanced cases of disease in hospital beds. _Hospital Treatment of Consumptives_