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 (Fig. 16-p). Although this had been known to take place in the amoebae 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; daphneae 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