No matter what medical science may say to the contrary, many priests hold that they have often left smallpox cases, for example, without disinfecting themselves, and that they have not spread the disease. This is a very rash assertion. It is absolutely certain that smallpox has been communicated to susceptible persons by those coming from patients ill with that disease merely passing the susceptible man on the street. The number of persons that will not take smallpox when exposed to it is very large. In Washington in 1895, during an epidemic of smallpox, 187 persons, to my personal knowledge, were exposed to one group of 39 smallpox patients without taking the disease. The unharmed had been present in sick-rooms or had even nursed the patients, not knowing that the disease was smallpox. In this epidemic eight persons lived in the same rooms with, or visited frequently, two patients that afterward died of virulent smallpox, and none of the eight took the disease. One of these eight, however, went into a dramshop, had one glass of beer and left immediately, and in fourteen days afterward (the average time of incubation) we took the barkeeper to the smallpox hospital. This barkeeper had not been exposed to smallpox except by contact with the man mentioned here. There were about 60 cases of smallpox in that epidemic, and we traced every one to direct or indirect contact with one initial case. If we were infected by every exposure to contagious disease the world would be depopulated. It is true that you cannot give some persons diphtheria if you actually put the Klebs-Loeffler bacillus into their mouths, and nurses and physicians in consumptive wards have the tubercle bacillus in their nostrils without ill effect. So for many diseases; but it unfortunately remains true that there are susceptible persons everywhere who will at once take a disease when they are exposed to it. {171} Immunity changes in the same person. Starvation, fatigue, loss of blood, unsuitable diet, exposure to heat, cold, and moisture, and other influences lessen the power of resistance to infection. Men vary almost as do the lower animals as regards infection. The quantity of tetanus toxin that will kill 400 horses will not bother a hen; Algerian sheep and the white rat are not affected by anthrax, but other sheep and the brown rat are very susceptible; a hog will not take glanders, man and a horse will; men, cattle, and monkeys have tuberculosis, dogs and goats do not; white men with few exceptions are susceptible to yellow fever and malaria, negroes are practically immune; negroes readily succumb to the fatal sleeping sickness, white men are almost immune; and similar differences are observable in the same race or family.