Sydenham defined disease as “an effort of Nature, striving with all her might to restore the patient by the elimination of morbific matter,” an implication of the modern idea of disease as a struggle for existence between invading micro-organisms and those factors residing in the body which make for immunity. The development of bacteriology emphasized the desirability of keeping the harmful germs away from the individual—of preventing disease by eliminating or restraining its causative agents. The accomplishments of public hygiene in this direction have undoubtedly been most significant; yet the hopes of many health officials have been somewhat disappointed because infectious ailments have not been eliminated in civilized communities. They have failed, however, to reckon with the innumerable unknown modes of invasion. Thus, the menace of the healthy carrier of infectious micro-organisms was scarcely recognized a generation ago; and, year by year, new sources of bacterial danger are being brought to notice. The great epidemics of meningitis or cerebrospinal fever, poliomyelitis, influenza and epidemic encephalitis, ravaging even the countries in which the medical sciences and protective arts have been most assiduously cultivated, have brought discouragement to many.