Civilization is only a thin crust, covering possibilities of relapse into savagery. Long ages have been required to bring civilized man by slow and painful stages to his present condition. Only a few months are required for him to relapse into the condition of barbarism from which he emerged. In times of great disaster, when the conventions of civilized society are removed, man again becomes an individualist, striving like primitive man or like the beasts of the jungle for food and shelter and even for life itself, and ruthlessly abandoning any refinements or customs which might impede him. Under such conditions, he easily reverts to the habits of his former savage state. Even the diseases of barbarism return.
A striking illustration is the reappearance of typhus fever in the war-swept countries of Europe. From the plague of Athens as described by Thucydides, down through the ages, typhus has always appeared