It would be misleading to infer that all of these benefits have been brought about solely through the collection of vital statistics, although much of it would have been difficult without the knowledge furnished by these statistics. Workers in almost every branch of pure science have contributed to the progress,—the physicist, the meteorologist, the chemist, and by no means the least, the biologist. Indeed, with the more recent investigations, the culture tube of the biologist has almost revolutionized medicine and all that pertains to it. Sanitary science seeks to accomplish two ends; it purposes to _prevent_ disease and to _promote_ public health. If it seeks to prevent disease, after the fashion of the oft-quoted cook-book, it must first secure the disease, or what is essentially the same thing, know what causes it. If the cause be known, and we can conquer the cause, we can prevent the disease. Thus a disease known as _trichinæ spiralis_, from the name of the parasite invading the body and causing sickness and death, is caused by eating pork infected by the trichinæ. We can certainly prevent trichinæ in persons by forbidding pork; but we also know that the trichinæ do not occur in all pork, and that their presence can be detected by the microscope. If, then, a sample from every slaughtered pig is submitted to the microscopist, the infected pork can be discovered. This is done in our large packing establishments, especially for that pork which is to be exported. Again, a thorough cooking will kill the trichinæ, even if present. Only the grossest carelessness, consequently, can account for a case of trichinæ, and, indeed, it is a very rarely occurring disease. This illustrates the importance of a knowledge of the cause of the disease, to enable one to devise a method for preventing it. In the study of disease causes, the biologist has been very successful during the past few years, and a number of our communicable diseases are demonstrated to be caused by the growth and development of bacteria. From this demonstration in the case of some, a general hypothesis has been formulated, which is useful as a working hypothesis, but by no means safe to call a theory as yet. This hypothesis is that all of our communicable diseases are caused by living organisms originating in one person and conveyed to another, where they begin to grow, to reproduce their kind and to perform their life functions. Hence all communicating diseases are infectious. Some of these infectious diseases, like measles or smallpox, are capable of direct communication from one person to another, rendering them contagious; others, like typhoid fever and cholera, are not contagious in this sense of the word. This is a very excellent distinction to make in the use of these much abused words.