There is a primary focus of disease from which the extension takes place, and the study of the modes of extension in the individual throws some light on the much more difficult subject of the transmission of disease from one individual to another.

There are four ways by which extension in the individual may take place.
1.

By continuity of tissue, an adjoining tissue or organ becoming infected by the extension of a focus of infection.
2.

By means of lymphatics.

Organisms easily enter these vessels which are in continuity with the tissue spaces and receive the exudate from the focus of infection.