The present-day problems in tuberculosis which can be approached by experimental or at least by laboratory methods manifest themselves in three different ways:
In the somewhat chaotic condition of opinion concerning the avenues through which tubercle bacilli gain a foothold in the body.
In the wide divergence of opinion concerning the relation of bovine to human tuberculosis; and
In the general trend of studies toward the problem of specific immunity, with special reference to prevention and treatment.
These three problems, though distinct, are interrelated, and in a lecture of this kind, in which some freedom in the statement of theories and hypotheses and a rather broad treatment of the subject are not only permissible but desirable, it must be necessary to deal with each, to some extent at least. The most important of the three is the one dealing with immunity, and my statements will be grouped around and directed