Disease and Its Causes

by W. T. Councilman

From the lungs the bacilli are carried by the lymphatics to the lymph nodes at the root of the lungs, in which a similar process takes place; this, on the whole, is favorable, because further extension by this route is for a time blocked. The extension by means of surfaces continues, the abundant sputum which is formed in the lungs and which contains large numbers of bacilli, becomes the vehicle of transportation. The windpipe and larynx may become infected, the back parts of each are more closely in contact with the sputum and are the parts most generally infected. A large part of the sputum is swallowed and infection of the intestine takes place, the lesions taking the form of large ulcers. From the intestinal ulcers there is further extension by means of the lymphatics, to the large lymph nodes in the back of the abdominal cavity (Fig. 8-25); the bacilli may also pass from the ulcers into the abdominal cavity and be distributed over the surface of the peritoneum resulting in tuberculous peritonitis. When the disease has reached an advanced stage, bacilli in small numbers continually pass into the blood and are distributed by this over the body, producing small nodules in many places. In rare cases distribution by the blood is the principal method of extension, and immense numbers of small foci of disease are produced, the form of disease being known as acute miliary tuberculosis. Although the bacilli are distributed everywhere, certain organs, as the brain and muscles, are usually exempt, because in these the conditions are not favorable to further growth of the bacilli. Tuberculosis, although frequently a very acute disease, is usually one of the best types of a chronic disease and may last for many years. The chronic form is characterized by periods of slow or rapid advance when conditions arise in the body favorable for the growth of the bacilli, and periods when the disease is checked and quiescent, the defensive forces of the body having gained the upper hand. Often the intervention of some other disease so weakens the defences of the body that the bacilli again find their opportunity. Thus typhoid fever, scarlet fever and other diseases may be followed by a rapidly fatal advance of the tuberculosis, starting from some old and quiescent focus of the disease.

Tuberculosis is also one of the best examples of what is known as latent infection. In this the infectious organisms enter the body and produce primary lesions in which the organisms persist but do not extend owing to their being enclosed in a dense and resistant tissue, or to the production of a local immunity to their action. Dr. Head has recently examined the children of households in which there was open tuberculosis in some member of the household. By open tuberculosis is understood a case from which bacilli are being discharged. He found with scarcely an exception that all the children in such families showed evidences of infection. The detection of slight degrees of tuberculous infection is now made easy by certain skin reactions on inoculation of the skin with a substance derived from the tubercle bacilli. Such latent infections may never become active and in the majority of cases do not. When, however, in consequence of some intercurrent disease or conditions of malnutrition the general defences of the body become weakened extension follows. Such latent infections explain the enormous frequency of tuberculosis in prisons. Under the general prison conditions infection in the prisons probably does not take place to any extent, and the disease is as common when the prisoners are kept in individual cells as in common prisons. It is probable that in these cases the prisoners have latent tuberculosis when entering, and the disease becomes active under the moral and physical depression which prison life entails.

For the extension of infection from one individual to another the infecting organisms must in some way be transferred. The most important of the conditions influencing this are the localization of the disease and the character of the infectious organisms, particularly with regard to their resistance to the conditions met with outside of the body. The seat of disease influences the discharge of organisms; thus, if the disease involve any of the surfaces the organisms become mingled with the secretions of the surface and are discharged with these. If the seat of disease be in the lungs, the throat or the mouth, the sputum forms the medium of extension, which can take place in many ways. The sputum may become dried, forms part of the dust and the organisms enter with the inspired air. The organisms which cause most of the diseases in which the sputum becomes infectious are quickly destroyed by conditions in the open, such as the sunlight and drying; street dust does not play so prominent a part in extension as is generally supposed. Organisms find much more favorable conditions within houses. It is now generally recognized that infection with tuberculosis does not take place in the open, but in houses in which the bacilli on being discharged are not destroyed. The hands, the clothing and surroundings even with the exercise of the greatest care may become soiled with the saliva.

It has been shown that in coughing and speaking very fine particles of spray are formed by the intermingling of air and saliva, which may be projected a considerable distance and remain floating in the air for some time. These particles are so fine as to be invisible; they may be inspired, and their presence in the air forms an area of indeterminate extent around the infected person within which such infection is possible. Such spray formation is also an important means of the extension of infection in the sick individual, for it is continually formed and inspired. It is in this way that the extreme prevalence of broncho-pneumonia in infants and young children is to be explained. No matter what the essential disease, an almost constant finding in young children after death is small areas of inflammation in the lungs in and around the terminations of the air tubes. The situation renders it evident that the organisms which caused the lesions entered the lung by the air tubes. The mouth of the child is unclean and harbors numbers of the same sort of organisms as those causing the lung inflammation; but in the absence of such a mode of infection as is given by spray formation it is difficult to see how the extension from the mouth to the lungs could take place. The weakened condition of the body in these cases favors the secondary infection.

If the disease be located in the intestines, as in typhoid fever and dysentery, the organisms are contained in the fecal discharges, and by means of these the infection is extended. In typhoid fever, dysentery and cholera massive infections of the populace may take place from the contamination of a water supply and the disease be extended over an entire city. One of the most striking instances of this mode of extension was in the epidemic of cholera in Hamburg in 1892. There were two sources of water supply, one of which was infected, and the cases were distributed in the city in the track of the infected supply. Many such instances have been seen in typhoid fever. Certain articles of food, particularly milk, serve as sources of infection. This is more apt to happen when the organism causing the infection grows easily outside of the body. A few such organisms entering into the milk can multiply enormously in a few hours and increase the amount of infectious material. In all these cases the sick individual remains a source of infection, for it is almost impossible to avoid some contamination of the body and the immediate surroundings with the organisms contained in the discharges.

Transmission by air plays but little part in the extension of infection. In such a disease as smallpox, where the localization is on the surface of the body, the organisms are contained in or on the thin epithelial scales which are constantly given off. These are light, and may remain floating in the air and carried by air currents just as is the pollen of plants. There seem to have been cases of smallpox where other modes of more direct transmission could be excluded and in which the organisms were carried in the air over a considerable space. All sorts of intermediate objects, both living and inanimate, such as persons, domestic animals, toys, books, money, etc., can serve as conveyors of infection.

Insects play a most important part in the transmission of disease, and in certain cases, as when a disease is localized in the blood, this is the only means of transmission. There are three ways in which the insect plays the rôle of conveyor.

1. The insect may play a purely passive part in that its exterior surface becomes contaminated with the discharges of the sick person, and in this way the organisms of disease may be conveyed to articles of food, etc. The ordinary house fly conveys in this way the organisms of typhoid and dysentery. Flies seek the discharges not only for food, but for the purpose of depositing their eggs, and the hairy and irregular surface of their feet facilitates contamination and conveyance. When flies eat such discharges the organisms may pass through the alimentary canal unchanged and be deposited with their feces; they also often vomit or regurgitate food, and in this way also contaminate objects. Flies very greedily devour the sputum of tuberculous patients, and the tubercle bacilli contained in this pass through them unchanged and are deposited in their feces.

Fig. 19.—Trypanosomes From Birds. All the trypanosomes are very much alike. They contain a nucleus represented by the dark area in the centre and a fur-like membrane terminating in a long whip-like flagellum. They have the power of very active motion within the blood.