This chapter discusses the possible influence of heredity in producing a congenital susceptibility. A special susceptibility to infection, hereditary or acquired, is generally regarded as appertaining to those who become tuberculous, and as being indeed necessary for the development of tuberculosis when infection is received. In those showing this special susceptibility vital resistance to invasion by disease is supposed to be deficient, or the patient is said to be abnormally vulnerable to disease. The difficulties of estimating resistance are particularly great in a disease which is so prevalent as tuberculosis. The error of this assumption can be seen by ascertaining what happens when a given number of persons are exposed to the infection of acute infectious diseases like scarlet fever, diphtheria, and enteric fever. The fact that old localised and cured tuberculous lesions are so often found at autopsies does not appear to indicate that the majority of the populations are naturally immune to tuberculosis.