Disease and Its Causes

by W. T. Councilman

The infectious diseases are often complicated by secondary infections, some other organism finding opportunity for invasion in the presence of the injuries produced in the primary disease. In many diseases, such as diphtheria, scarlet fever and smallpox, death is frequently due to the secondary infection. The secondary invaders not only find local conditions favoring a successful attack, but the activity of the tissue cells on which the production of protective substances essentially depends has suffered by the primary infection, or the cells are occupied in meeting the exigencies of this. The body is in the position of a state invaded by a second power where all its forces and resources are engaged in repelling the first attack.

What are known as terminal infections occur shortly before death. No matter what the disease which causes death, in the last hours of life the body usually becomes invaded by organisms which find their opportunity in the then defenceless tissues, and the end is often hastened by this invasion.

There are also mixed infections in which two different organisms unite in attack, each in some way assisting in the action of the other. The best known example of this is in the highly infectious disease of swine known as hog cholera. It has been shown that in this disease two organisms are associated,—one an invisible and filterable organism, and the other a bacillus. It was first supposed that the bacillus was the specific organism; it was found in the lesions and certain, but not all, the features of the disease were produced by inoculating hogs with pure cultures. The disease so produced is not contagious, and the contagious element seems to be due to the filterable virus.

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.

4. By means of continuous surfaces. The bacteria may either grow along such surfaces forming a continuous or more or less broken layer, or may be carried from place to place in the fluids which bathe them.

All these modes of extension are well shown in tuberculosis. This disease is caused by a small bacillus which does not produce spores, has no power of saphrophytic growth under natural conditions, and is easily destroyed. Moisture and darkness are favorable conditions for its existence, sunlight and dryness the reverse. 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. The examination of two hundred and twenty-five children of the average age of five years who had died of diphtheria showed tuberculous infection in one-fifth of the cases and the frequency of infection increases with age. The defence on the part of the body is chiefly by the formation of dense masses of cicatricial tissue which walls off the affected area and in which the bacilli do not find favorable conditions for growth. This mode of defence, which is probably combined with the production of substances antagonistic to the toxines produced by the bacilli, is so efficacious that in the great majority of cases no further extension of the process takes place. In certain cases, however, the growth of the bacilli in the focus is unchecked, the tissue about them is killed and becomes converted into a soft semi-fluid material; further extension then takes place. All parts of the enormous surface of the lungs are connected by means of the system of air tubes or bronchi, and the bacilli have favorable opportunity for distribution, which is facilitated by sudden movements of the air currents in the lung produced by coughing. The defence of the body can still keep pace with the attack, and even in an advanced stage the infection can be checked in some cases permanently; in others the check is but temporary, the process of softening continues, and large cavities are produced by the destruction of the tissue. On the inner surface of these cavities there may be a rapid growth of bacilli.