The sources and modes of infection
This volume is intended to indicate the principles which should guide sanitary practice, and to show how recent laboratory work and the epidemiological study of disease have modified these principles. When I began work as health officer in 1884 the filth theory was still in favor, and it was generally believed that the germs of disease commonly grew in decaying organic matter. Yet contagion was recognized as an important factor in the spread of disease, and the isolation of the sick was more and more insisted upon. Fifteen years ago probably most health officials believed that the contagious diseases could be completely stamped out if only all persons sick with them could be isolated. The air was thought to be the chief medium for their transmission, and fomites the mechanism for their passage from place to place. Sanitary practice was based on these premises. My own views concerning these matters became greatly modified year by year, partly owing to the rapidly accumulating knowledge of bacteria and other disease-producing organisms, and partly owing to direct observations on the manner in which the infectious diseases are disseminated, and on the effect of preventive measures. It now appears that the growth of disease germs outside of the body is not frequent enough to be an important factor in the causation of disease, but their growth in the body without causing sickness, their latency as it were, often for many months, is a factor of very great significance. We know now that direct contact with the sick, or with healthy carriers of disease germs, is an exceedingly frequent mode of transmission, and that infection by means of the air, or from infected articles, is not nearly as common as was formerly believed.