Gresswell in 1886 brought forward certain facts which appeared to show that diphtheria in certain individuals may become a chronic disease, and from time to time enter upon an active and infectious phase. The author collected similar evidence of cases of diphtheria, and occasionally also of scarlet fever, in which the infection persisted for very long periods, and subsequently reappeared after intervals of considerable length. The analogy between these exceptional conditions and tuberculosis is obvious. The author illustrates prolonged latency between the last known exposure to infection and the occurrence of an attack of pulmonary tuberculosis. The clinical occurrence, both in tuberculosis and in other infective diseases, of prolonged secondary latency. The lesion may become encapsulated and so remain for years without producing manifest clinical symptoms. Tuberculous lesions may have long periods of latency in animals and man. The clinical evidence of this phenomenon in tuberculosis and in diphtheria is strongly confirmed by bacteriological evidence concerning other diseases.