arise in the body favorable for the growth of the bacilli, and periods
when the disease is checked and quiescent, the defensive forces of the
body having gained the upper hand. Often the intervention of some
other disease so weakens the defences of the body that the bacilli
again find their opportunity. Thus typhoid fever, scarlet fever and
other diseases may be followed by a rapidly fatal advance of the
tuberculosis, starting from some old and quiescent focus of the
disease.

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