It is commonly neither wise nor necessary for people not professionally concerned to think much about disease, or weigh anxiously the chance or mode of its acquirement. But now and then conditions arise which demand general attention and instruction regarding certain diseases in order that a great, threatening calamity may be averted. Such a condition faces the people in all lands to-day in the appalling prevalence of tuberculosis. A disease which in mild or severe form affects at least one-half of the whole human race, and which causes the death of full one-seventh of all who pass away, killing about one-third of those who perish between the ages of fifteen and forty-five--a disease which is most insidious in its onset, and often relentless in its course, and which may be largely prevented--is one about which we cannot be indifferent, and should not be longer inactive. There has long been reason for believing that tuberculosis is a communicable disease. Its prevalence in certain families and communities, its frequent occurrence in those who have personally attended upon its victims, its onset in those who have occupied apartments vacated by consumptives--such facts observed over and over again abundantly justify the belief in its communicability. Up to the commencement of the last decade the cause of the disease was altogether unknown, and no definite data were at hand which could enable us to fix upon a feasible plan for limiting its ravages. But in these later years a great light has been thrown upon this and other kindred diseases. Most intelligent people are aware that within the past decade a new field in the domain of life has been revealed and widely explored. It has been learned that in earth and air and water there exist countless myriads of living things so minute as to lie far beyond the limits of the unaided vision, and yet in the aggregate so potent in the maintenance of the cycle of life upon earth that without their activity all life would soon cease to be, and the elements which for a short span fall under the sway of the life forces in all higher animals and plants would lapse finally and irrevocably into their primal state. These tiny organisms are called germs, microbes, or micro-organisms. One great and important group of them belongs among the microscopic plants called bacteria. These bacteria as a class are important in their economy of nature, because they live for the most part on dead organic material--that is, such material as has once formed a portion of some living thing.