IV. RESEARCH NEEDED TO LEARN HOW TO COPE WITH AIRBORNE INJECTION.

Abstract

IT is unquestionably presumptuous to deal with the subject assigned to me, not only because it is a tremendously large topic whose solution has been the focus of many of our leading investigators, but also because of the nature of modern biological research. Since the growth of knowledge is stepwise with new concepts developing from previous experiments, it is not possible to spell out a program except in the broadest terms. However, it may be possible to relate what has been accomplished with some diseases to what needs be done in others, and in addition, since the situation is less than ideal, even among the best controlled diseases, to speculate about the future. As a framework for our thoughts we will use the familiar circular pathway, shown in Figure 1, often used for teaching these diseases. Since the principle focus of the topic is the method of spread, it is used as the starting point. Two types of spread are recognized based on the classification of Wells: droplet nuclei or true airborne particles which do not settle before ventilation and dustborne material, the residuals of particles which are large enough to settle before ventilation can remove them. The data obtained on the study of tuberculosis and measles, the two best studied true airborne diseases, indicate how different the gross epidemiologic pattern can be in spite of the common mechanisms. These differences depend on factors such as the density of infectious particles generated by the source case and the susceptibility of the exposed population, but illustrate that much research must be done in various populations before conclusions can be reached about the airborne spread of newly discovered agents. It is of interest, for example, that even though the superficial aspects of tuberculosis epidemiology suggest direct transmission by contact, and possibly indirectly by fomites and dust, it is probably rarely transmitted except by droplet nuclei, the prolonged contact being required because of the low density of infectious nuclei. This need for prolonged indoor proximity serves as a basis for much of the control work based upon case finding among close contacts. It is clear that since there is only one infectious unit in 20,000 cubic feet of air, several months of living in the environment are required on the average to infect a single individual. On the other hand, if the number of susceptibles exposed to the same volume of similarly infected air is markedly increased, as in schoolrooms, public eating places, public transportation, and so on, some will be infected much earlier. The patients with unexplained infections may come from a tremendously large population base and thus represent the very small percentage who become infected by short sojourn in infected air. As tuberculosis becomes more of a vagabond's disease, partly due to successful programs based on control of known contacts, the proportion of those with unknown source should increase. The fact that direct contact need not be used to explain the epidemiologic pattern of tuberculosis indicates the need for details about the portal of entry, hence the optimal size of the infectious quantum, their frequency in air, and so forth before definite decisions can be made about mode of spread. It is likely, however, that when the portal of entry re-