There are three varieties or strains of the tubercle bacilli which infect respectively man, cattle and birds, and each class of animals shows considerable resistance to the varieties of the bacillus which are most infectious for the others.
The primary seat of the infection in man is generally in the upper part of the lung.

The organisms settle on the surface here and cause multiplication of the cells and an inflammatory exudate in a small area.

With the continuous growth of the bacilli in the focus, adjoining areas of the lung become affected, and there is further extension in the immediate vicinity by means of the lymphatics.

Small nodules are formed and larger areas by their coalescence.

Infection with tuberculosis is so common that at least three-fourths of all individuals over forty show evidences of it.