It is evident that just as the parasite has his weapons of offence and defence so has the host, otherwise there would be no recovery from infectious diseases. Although many of the infectious diseases have a high mortality, which in rare instances reaches one hundred per cent, the majority do recover. In certain cases the recovery is attended by immunity, the individual being protected to a greater or less degree from a recurrence of the same disease. The immunity is never absolute; it may last for a number of years only, and usually, if the disease be again acquired, the second attack is milder than the primary. Probably the most enduring immunity is in smallpox, although cases are known of two and even three attacks; the immunity is high in scarlet fever, measles, mumps and typhoid fever. The immunity from diphtheria is short, and in pneumonia, although there must be a temporary immunity, future susceptibility to the disease is probably increased. In certain cases the immunity is only local; the focus of disease heals because the tissue there has evolved means of protection from the parasite, but if any other part of the body be infected, the disease pursues the usual course. A boil, for example, is frequently followed by the appearance of similar boils in the vicinity due to the infection of the skin by the micrococci from the first boil, which by dressings, etc., have become spread over the surface.

The natural methods of defence of the host against the parasites have formed the main subject in the study of the infectious diseases for the last twenty years. Speculation in this territory has been rife and most of it fruitless, but by patient study of disease in man and by animal experimentation there has been gradually evolved a sum of knowledge which has been applied in many cases to the treatment of infectious diseases with immense benefit. Research was naturally turned to this subject, for it was evident that the processes by which the protection of the body was brought about must be known before there could be a really rational method of treatment directed towards the artificial induction of such processes, or hastening and strengthening those which were taking place. Previous to knowledge of the bacteria, their mode of life, their methods of infection and knowledge of the defences of the body, most of the methods of prevention and treatment of the infectious diseases was based largely on conjecture, the one brilliant exception being the discovery of vaccination by Jenner in 1798.

The host possesses the passive defences of the surfaces which have already been considered. The first theories advanced in explanation of immunity were influenced by what was known of fermentation. One, the exhaustion theory, assumed that in the course of disease substances contained in the body and necessary for the growth of the bacteria became exhausted and the bacteria died in consequence. Another, the theory of addition, assumed that in the course of the disease substances inimical to the bacteria were formed. Both these theories were inadequate and not in accord with what was known of the physiology of the body. The most general mode of defence is by phagocytosis, the property which many cells have of devouring and digesting solid substances. Although this had been known to take place in the amoebæ and other unicellular organisms, the wide extent of the process and its importance in immunity was first recognized by Metschnikoff in 1884 and the phagocytic theory of immunity advanced and defended by a brilliant series of experiments by Metschnikoff and his pupils conducted in the Pasteur Institute. Metschnikoff's first observations were made on the daphnea, a small animalcule just visible to the naked eye which lives in fresh water. The structure of the organism is simple, consisting of an external and internal surface between which there is a space, the body cavity; daphneæ are transparent and can be studied under the microscope while living. Metschnikoff observed that certain of them in the aquarium gradually lost their transparency and died, and examining these he found they were attacked by a species of fungus having long, thin spores. These spores were taken into the intestine with other food; they penetrated the thin wall of the intestine, passed into the body cavity, multiplied there, and in consequence the animal died. In many cases, however, those penetrating became enclosed in cells which the body cavity contains and which correspond with the leucocytes of the blood; in these the spores were digested and destroyed. The daphneæ in which this took place recovered from the infection. Here was a case in which all the stages of an infectious disease could be directly followed under the microscope, and the whole process was simple in comparison with infections in the higher animals. The pathogenic organism was known, the manner and site of invasion was clear, it was also evident that if the multiplication of the parasite was unchecked the animal died, but if the parasite was opposed by the body cells and destroyed the animal recovered. The studies were carried further into

the diseases of the higher animals, and it was found the leucocytes in these played the same part as did the cells in the body cavity of the daphnea. The introduction of bacteria into certain animals was followed by their destruction within cells and no disease resulted; if this did not take place, the bacteria multiplied and produced disease. Support also was given the theory by the demonstration at about the same time that in most of the infectious diseases the leucocytes of the blood became increased in number, - that in pneumonia, for instance, instead of the usual number of eight thousand in a cubic millimeter of blood, there were often thirty thousand or even fifty thousand. At about the same time also chemotaxis, or the action of chemical substances in attracting or repelling organisms, excited attention, and all these facts together became woven into the theory. It was soon seen, however, that this theory, based as it was on observation and supported by the facts observed, was not, at least in its first crude form, capable of general application. Many animals have natural immunity to certain diseases; they do not have the disease under natural conditions, nor do they acquire the disease when the organisms causing it are artificially introduced into their tissues by inoculation. Such natural immunity seemed to be unconnected with defence by phagocytosis, for the leucocytes of the animal might or might not have phagocytic reaction to the particular organisms to which the animal was immune. It was also seen that recovery from infection in certain diseases was unconnected with phagocytosis. It had also been demonstrated, by German observers chiefly, that the serum of the blood, the colorless fluid in which the corpuscles float, was itself destructive, and that in an animal rendered immune to a special bacterium the destructive action of the serum on that organism was greatly increased. In this hostile serum the bacteria often became clumped together in masses, the bodies became swollen, broken up, and finally disintegrated. This property of the serum was described as due to a substance in the serum called alexine, which in the immune animal became greatly increased in amount. It was even denied by some that phagocytosis of living bacteria took place, and that all those included in the cells were dead, having been destroyed in the first instance by the serum.

New York, Henry Holt And Company.
London, Williams and Norgate, originally published 1913

Tags: Infectious Diseases