Despite the great amount of information that has been assembled on the nature of infectious disease, many fundamental problems remain unsolved. Among others, the question of just how existing balances between micro-organism and host are upset so as to lead to the spread of infection in epidemic form, demands explanation. It is clear, for example, that all cases of lobar pneumonia may be traced back ultimately to a previous case or to a pneumococcus carrier, but just how and why such a carrier will give rise to disease at one time and be harmless at another, is quite obscure. Similar considerations apply to other infections.
The lack of exact information on this question may be due in a measure to the mode of attack which has been employed; when clinical epidemiologic studies have been adequate, exact bacteriologic control has usually not been available, and, vice versa, exact bacteriologic studies have