In studying the ætiology of any ailment it is profitable to assume that no disease has a single cause. All the manifestations which constitute disease are the result of a disturbance of equilibrium between the human organism and its environment. In the production of this disharmony many factors play a part, each varying in importance from case to case. Nevertheless, since the discoveries of Pasteur we have become accustomed to view diseases mainly from the standpoint of their causative organisms, thereby neglecting many equally important factors in their ætiology. Yet we know of many instances where these selfsame pathogenic agents reside in the human body without producing any symptoms of disease. The meningococcus and diphtheria bacillus are found in the air-passages of many healthy people, while after the disease has run its course, the causative organism in typhoid may continue to occupy the body indefinitely. Moreover it frequently happens that living parasites of malaria, purposely introduced into the human subject, fail to produce any symptoms after many weeks of incubation. In cases of this nature it may transpire that the addition of further noxious influence, such as an injection of milk or even a cold bath, may precipitate the symptoms of malaria, which thereafter runs its normal course. Conversely, there are many individuals, or groups of individuals, who are particularly prone to fall victim to maladies from which others appear to be protected. The mentally disordered are very liable to infection by the Flexner group of organisms, while those who nurse them are seldom attacked. Such illustrations, which might be multiplied indefinitely, serve as a useful reminder of the complex factors at work in the production of diseases of microbic origin. The soil is just as important as the seed, and no study of ætiology would be complete were the question of individual susceptibility to be omitted.