A Study of the Factors Determining Infection in Animals.* by Charles

The discovery of micro6rganisms and their identification as causative agents in the production of specific diseases has led in recent years to the newer science of immunology. It may be stated that our present conception of the defensive and offensive mechanism of the host toward parasitic invasion is the direct outcome of studies upon the biochemical properties of bacteria and protozoa. Furthermore, the cultivation and study of the biological characteristics of microSrganisms are of the greatest value in rendering possible the study of immune processes. Immunology has presented in particular three problems for elucidation: ( I ) t h e factors determining the infection of man and animals, and the development of changes in the tissues--the result of bacterial and protozoal action; (2) the method through which parasites exert their detrimental influence, whether by toxin or simply by mechanical injury to the tissues; and (3) the factors determining the manner of production of protective and curative substances on the part of the infected animal. Naturally, in such a study we must take into consideration the possibility of protective properties on the part of the invading parasites; that is, their immunity against injurious substances already present or subsequently produced in the body of the