If disease germs are received by a person whose body acts as barren
soil no disease results, while if they reach a person whose body
furnishes suitable soil for growth, disease follows. The former
person is said to be immune to the disease. The latter is spoken of
as susceptible. It is well known that few persons ever have the same
contagious disease twice. One attack renders immune a person who
was susceptible. This explains why we have epidemics of contagious
diseases. During the epidemic most of the persons in a community who
are susceptible contract the disease. At the end of the epidemic most
of the suitable soil for the growth of the special germ has been
exhausted and the population of the community has become immune to the
disease. This causes the epidemic to cease. Another outbreak in the
same community of the same disease can only occur when susceptible
individuals have again accumulated, i. e., when children have been
born and reached a suitable age. Epidemics of contagious diseases are
naturally confined largely to children who have grown up after the
last epidemic occurred, the older members of the community having been
rendered immune by attack of the disease earlier in life. In isolated
situations where the germs of contagious diseases are not often
introduced persons may reach adult age without ever having been exposed
to them. Such adults may then become infected the same as children. In
the late world war large numbers of young men from rural communities
who had never had the usual contagious diseases were brought together
in training camps, and thus furnished fertile soil for many epidemic
diseases. In a study of over 30,000 native white children in 14
localities in the United States it was found that at 5 years of age
65% have had measles, 48% whooping cough, 22% chicken pox, 5% scarlet
fever, and 3.5% diphtheria. As age advanced the proportions increased
until at 15 years of age, 88% have had measles, 77.6% whooping cough,
51% chicken pox, 11.6% scarlet fever, and 8.7% diphtheria. By young
adult age most persons have had these diseases which are often spoken
of as children’s diseases, but some persons are affected later, some
adults at quite advanced age.