The modes of transmission of infectious diseases are of great importance and are the foundation of measures of public health.  In the preceding chapter we have seen that in the infected individual the disease extends from one part of the body to another.  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.  The organisms are carried to the lymph nodes, which, acting as filters, retain them and for a time prevent a further extension.  The following illustrates the importance of the part the nodes may play in mechanically holding back a flood of infection.  A physician examined after death the body of a person who died from infection with a very virulent micrococcus and in the course of the examination slightly scratched a finger.  One of the organs of the body was removed, sent to a laboratory and received by a laboratory worker, a woman physician, who had slight abrasions and fissures in the skin of the hands from contact with irritating chemicals.  In the course of a few hours the wound on the finger of the man became inflamed, intensely painful, and red lines extended up the arm in the course of the lymphatic vessels, showing that the organisms were in the lymphatics and causing inflammation in their course.  The lymph nodes in the armpit into which these vessels empty became greatly inflamed, swollen, and an abscess formed in them which was opened.  There was high fever, great prostration, a serious illness from which the man did not recover for several months.  The woman only handled the organ which was sent to the laboratory in order to place it in a fluid for preservation.  She also had a focus of infection of a finger with the same red lines on the arm, showing extension by the lymphatics; but there was no halt of the infection in the armpit, for all the lymph nodes there had been removed several years before in the course of an operation for a tumor of the breast.  A general infection of the blood took place, there was very high fever, and death followed in a few days.  The halt of the infection is important in allowing time for the body to make ready its means of defence.  One cannot avoid comparing the lymph node with a strong fortress thrown in the path of a victorious invading army behind which the defenders may gather and which affords them time to renovate their strength.

3.  By means of the blood.  The blood vessels are universally distributed, the smaller vessels have thin walls easily ruptured and easily penetrated.  It is probable that in every infection some organisms enter the blood which, under usual conditions, is peculiarly hostile to bacteria.  These may, however, be carried by the blood to other organs and start foci of infection in these.