Tuberculosis is also one of the best examples of what is known as latent infection. In this the infectious organisms enter the body and produce primary lesions in which the organisms persist but do not extend owing to their being enclosed in a dense and resistant tissue, or to the production of a local immunity to their action. Dr. Head has recently examined the children of households in which there was open tuberculosis in some member of the household. By open tuberculosis is understood a case from which bacilli are being discharged. He found with scarcely an exception that all the children in such families showed evidences of infection. The detection of slight degrees of tuberculous infection is now made easy by certain skin reactions on inoculation of the skin with a substance derived from the tubercle bacilli. Such latent infections may never become active and in the majority of cases do not. When, however, in consequence of some intercurrent disease or conditions of malnutrition the general defences of the body become weakened extension follows. Such latent infections explain the enormous frequency of tuberculosis in prisons. Under the general prison conditions infection in the prisons probably does not take place to any extent, and the disease is as common when the prisoners are kept in individual cells as in common prisons. It is probable that in these cases the prisoners have latent tuberculosis when entering, and the disease becomes active under the moral and physical depression which prison life entails. For the extension of infection from one individual to another the infecting organisms must in some way be transferred. The most important of the conditions influencing this are the localization of the disease and the character of the infectious organisms, particularly with regard to their resistance to the conditions met with outside of the body. The seat of disease influences the discharge of organisms; thus, if the disease involve any of the surfaces the organisms become mingled with the secretions of the surface and are discharged with these. If the seat of disease be in the lungs, the throat or the mouth, the sputum forms the medium of extension, which can take place in many ways. The sputum may become dried, forms part of the dust and the organisms enter with the inspired air. The organisms which cause most of the diseases in which the sputum becomes infectious are quickly destroyed by conditions in the open, such as the sunlight and drying; street dust does not play so prominent a part in extension as is generally supposed. Organisms find much more favorable conditions within houses. It is now generally recognized that infection with tuberculosis does not take place in the open, but in houses in which the bacilli on being discharged are not destroyed. The hands, the clothing and surroundings even with the exercise of the greatest care may become soiled with the saliva.