So far we have tried to explain the manner of origin of the most important diseases of childhood. Perhaps it would be better to consider how they spread. We still have the other question to answer; namely, How does the course of disease differ in childhood from that seen in adult life? The Course of Disease in have shown how most diseases arise from bacteria. from what has been said we that disease is the expression of the reaction of the human being to the ex citing cause of disease. Practical experience has taught us that these diseases do not appear immediately, but follow the supposed inocu lation of the organism. Hours, days, and in some eases even weeks go by before the clinical symptoms appear. This interval. which extends from the time of infection to the appearance of the first symptoms, is known as the "incubation period." This period of incubation, more or less long, which appears in each infectious disease, is explained oy the need of the specific or exciting cause of the disease when in the body, for a certain time in which to develop so that the toxins may be suffi cient in amount to produce symptoms (a reaction). An initial ] esis lance belonging to the organism must first be overcome, in order that the exciting cause may be felt and produce a demonstrable reaction in the form of a disease. This hypothesis may be plausible, but it is not satisfactory when considered in a critical light.
The theories and experimental investigations of von Pirquet and Schick arc of much service in the question under consideration. They demonstrate clearly how the symptoms following the injection of diph theritic scrum, scarlet fever serum, and even the pure horse serum, which they designate as the "serum disease," may be able to give us new fundamental ideas on the incubation period as well on the course of disease. These experiments show that when the organism has had time to manufacture specific antibodies for the especial acute infectious disease attacking it, the disease appears. They claim that the disease, shown by the appearance of symptoms, is nothing more or less than the reaction between the toxins and the antibodies. The time preceding this, after infection has occurred, during which the organism forms the antibodies, is the period of incubation. Von Pirquet and Schick claim it is the same for measles as it is for the serum exanthem; namely, that the first symptom is developed at the time when the organism has formed antibodies to resist the invading cause. In other words, it is the time when the antibodies, appearing free in the blood, react with the circu lating toxins of the disease, i.c., of measles. The formation of anti bodies appears, in this light, to be the signal for the appearance of the disease.
While this conception at the start seems to be improbable and para doxical, that the body is sick because it is fighting against the toxins and is producing substances which we have considered formerly were only of use to the body, yet it strikes one as quite probable and a very logical idea when looked at from a critical point of view.
We should like in this connection to draw attention to a discovery of Jelile, which has seldom been noticed. lie found that in acute pneu monia (luring childhood great numbers of agglutinins were formed, during the first days of the disease; they did not increase, however, but remained about the same, and during the crisis fell quickly. In the theory of von Pirquet and Schick. the agglutinins, being a rough esti mate of the amount of the antibodies present, were thought to increase in number at the time of the crisis, quite the opposite from the above findings of Jellle.
The formation of antibodies is peculiarly essential for the body. The affinity or disposition in man to react to the presence of the excit ing cause of pneumonia, measles, etc., compels him to produce anti bodies. The reactions between these and the toxins is seen in the ap pearance of the disease. The severity and duration of an illness depends on certain peculiarities in the course of the reaction (Ow intensity and duration).
The investigations of von Pirquet and Schick, most fundamental in the study of general pathology, are not the only supporting this view. Bail gives quite a number of experiments on animals, made by himself and his assistant. Infectious disease was exiwrimentally pro duced and symptoms quite similar to the above were seen. Bail also asserts that the formation of immune bodies is inevitable in animals.
The tuberculin reaction. which shows an especial susceptil)ility of the tubercular patient for certain poisons of the tubercle bacilli, finds a clear explanation in the theories of von Pirquet and Schick. A patient ill with tuberculosis who, at the time, is in an afebrile period. reacts on the injection of tuberculin by a rise of temperature. This appears be cause in the patient's blood antibodies exist which immediately react with the injected tubercular toxins.
As we have seen, the course of a disease depends on the intensity and duration of the reaction, which takes place between the immune bodies and the tissues. This produces a symptom-complex which we generally speak of as a tuberculin reaction. As long as the toxins are manufactured and the exciting causes find favorable conditions life in the organism, just so long will the disease exist. As soon as the cells essential for life stop acting, being damaged by the toxins, When the poisons are destroyed and no fresh.ones are formed because the bacteria are dead, recovery takes place. This recovery, however, only occurs if the organism has not been weakened too much by the dis ease, which often leads on to slow decline and even to death. This happens when the cells are so damaged that they are unable to recover front the harm clone by the toxins, as is illustrated in heart failure following diphtheria.