The almost universal prevalence of tuberculosis, the enormous death rate, together with its possible prevention and cure, have focused the attention of the world on this disease in a way and to an extent unique in the history of medicine. The opportunities for infection are so frequent, and the number of recoveries in which the healed lesions are recognized only postmortem are so numerous, as to justify the German-folk saying, "Jedermann hat am Ende ein bischen Tuberculose." It is evident, then, that some other element than that of infection merely must influence the course of this disease, otherwise, as almost every one is exposed to infection, the world would become a mortuary from this cause alone. It is generally recognized, however, that any condition which lowers the vital resistance or increases what Jonathan Hutchinson calls the "vulnerability," predisposes to infection and retards recovery.
Among the earliest symptoms of beginning tuberculosis