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The decline in the incidence of communicable diseases in recent years throughout Europe and America must be due in large part to (1) greater nonspecific resistance to disease, traceable to improvement in the physical condition of the population; (2) more widespread specific resistance as a result of active artificial immunization, and (3) improved methods of interrupting the spread of the causative agent from one person to another or of eradicating natural reservoirs of the infection. While all of these mechanisms have undoubtedly been contributory, the influence of each must have varied with the type of infection and with the portal of entry of the infecting agent. Considered in this light, communicable diseases may be classified into four groups, as shown in table 1. Some disorders may rightly fall into two separate classifications. The technic of prevention obviously has varied among these four groups. The control of the diseases in group
Charles F. McKhann (1938) studied this question.