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.