In the category of time, far out of human reach, there are circumstances which greatly influence contagion.... These almost cosmic arisings are spreadings of disease or facts of cosmo-chemical disturbance which no mere contagionism can explain. These words had special reference to cholera, and although we still know little or nothing of the mysterious influences which permit cholera when unimpeded to undertake transmundane travels at irregular intervals of time, we can claim with certainty that in any country in which sanitary surveillance is well organised, and the internal sanitation of the country is good, the spread of cholera need not be feared. Thanks to the great discovery of Jenner and to the complete organization of measures for isolation of the sick, and for vaccination and surveillance of contacts, we can make the same claim for smallpox, whenever this mysterious disease begins its occasional world travels. But we have to confess our continuing relative helplessness in preventing the spread of measles, and of acute catarrhs, among our endemic infections, and still more of influenza when—as recently—it makes its devastating swoop on the entire world, and secures a larger number of victims than the World War itself. We can recommend isolation of the sick, and personal precautions in speaking and in coughing and sneezing, and occasionally may score an isolated success; but we are practically helpless against this enemy. Nor are we better acquainted with the means for preventing the spread of poliomyelitis; and we cannot claim that any measure against the spread of cerebro-spinal fever has had undoubted success, except only rapid amelioration of the conditions of overcrowding under which it especially occurs. These instances suffice to show that in the region of respiratory infections,—with the one notable exception of tuberculosis, which we can control, whenever we are ready to take the necessary complete measures—we have much to learn. In respect of most diseases due to respiratory infection we are groping in darkness nearly as dense as that which beset Chadwick, Farr and Simon in their earlier work, and with little hope of any campaign comparable with that against dirt _en masse_, which was largely effective in reducing the specific infections of cholera, dysentery, and enteric fever, of typhus fever and even of tuberculosis. The great public health requirements for the future are the conquest over acute respiratory infections, including not only affections of the lungs, but probably also measles and whooping cough, cerebro-spinal fever and poliomyelitis and their allies; and the prevention of cancer. So while thankful for the discoveries already made, and for the beneficent work already accomplished, we must hope that the rapid increase of Medical Research in England and here will in due time enable us to extend the application of preventive medicine to diseases so far uncontrollable.