Tuberculosis, although frequently a very acute disease, is usually one of the best types of a chronic disease and may last for many years.

The chronic form is characterized by periods of slow or rapid advance when conditions 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.