The natural longevity of man is directly influenced by several factors and among the more important of these may be mentioned inheritance, environment, occupation and nutrition. Man has succeeded in his “struggle for existence” because of the development of agents of defense, external and internal. By means of his intelligence, he has been able to devise methods of defense and offense against external enemies. As the gift of his inheritance, he has developed internal defensive agents which are shared by the whole animal kingdom. These are many and elusive, exist in his tissues, and are both cellular and humoral in character. By their means, individually and racially, he has waged a defensive and offensive battle against infectious diseases; many individuals succumbing, Homo sapiens, as a species, surviving. To this end, environment, hygienic and climatic, is a contributing factor, as well as occupation, many forms of which predispose to disease.