How infection affects the body was the supreme mystery that the scientists of the past strove in vain to penetrate. By no devices of their laboratories could they detect the agents that caused the epidemic. There was only one satisfactory explanation of the origin and spread of the devastating plagues, which seemed to fall from the heavens on the people, and that was that epidemics were “a visitation of God” on account of the sins of the people. Of course, the only preventive and curative measure available and effectual was “repentance, prayer, and humiliation.” It is a cause of devout thankfulness that while these things were hidden from the “wise and prudent” of former times, they have in these latter days been revealed unto “babes.” No event in human history would have more greatly taxed the credulity of the most learned and experienced physician of half a century ago than the prophecy that in the early years of the twentieth century school children would be taught by simple and easily understood object lessons how to prevent and how to cure consumption, the Asiatic cholera, yellow fever, and other epidemics that have devastated cities, destroyed armies, and swept from the earth whole tribes of primitive people. But that prophecy has been literally fulfilled. During the last summer there has been a traveling object lesson that visited the different sections of the State of New York and taught the people, especially the children, all the essential facts as to the nature of the infection of tuberculosis, its effects on the body, and the methods of prevention and cure. [Sidenote: How Infection Works] As infective diseases cause the vast majority of cases of severe and crippling affections and of deaths in every community, the value of a knowledge of the nature of infection and how it affects the body, by the people of all ranks, ages, and conditions, cannot be estimated in its influence on the future of the human race. Already we learn that within the period referred to the sickness and death-rates of communities where the people have been most thoroughly instructed as to the nature of infective diseases, and how they affect the body, have greatly diminished, and the average human life has been markedly lengthened. Indeed, it now seems possible to restore the patriarchal age when a man may live to be “an hundred and twenty years old ... his eye ... not dim, nor his natural force abated.”