At this primary localization, the _atrium_ of infection,[1] the organisms multiply and from this point further invasion takes place.

Many secondary foci may be formed in the organs by distribution of the organisms, or there may be infection of the blood and fluids of the body.

The injuries which are produced depend upon the nature of the infecting organisms.

The most common lesion consists in the death of the tissue about the infecting organisms.

In most cases the sum of the changes are so characteristic that from them the nature of the infection is easily determined, and these changes often give names to the disease; thus tuberculosis is a disease characterized by the formation of tubercles or little nodules in the body.