Dr. Koch was the first to call attention to this danger when he discovered, during the Hamburg cholera epidemic, that _perfectly healthy_ persons were infected with cholera vibrios, and were the unconscious means of spreading the disease. Still more recently it has been found that true typhoid germs may similarly be present in persons not suffering from typhoid fever but sharing the same living-rooms. Huxley has said "science is nothing but trained and organised common sense," and it is in this spirit that we must endeavour to make use of the discoveries which have been made in the prevention of disease, in which the science of bacteriology has played so great and important a part. SUNSHINE AND LIFE It was nearly a century ago that a German physician incidentally wrote, "Our houses, hospitals, and infirmaries will, without doubt, some day be like hot-houses, so arranged that the light, even that of the moon and stars, is permitted to penetrate without let or hindrance." This was spoken long before the world of micro-organisms had been discovered, but curiously has found an echo in the writings of a distinguished bacteriological chemist in recent years. "Laissons donc entrer largement partout l'air et le soleil," writes M. Duclaux; "c'est là une maxime bien ancienne, mais si les mots sont vieux l'idée qu'ils revêtent est nouvelle." The interpretation of this ancient maxim is indeed very modern, and we must turn to the investigations made within the past few years to learn with what justification M. Duclaux thus expresses himself, for it is only comparatively recently that we have learnt the novel fact that sunshine, whilst essential to green plant life, is by no means indispensable to the most primitive forms of vegetable existence with which we are acquainted, _i.e._ bacteria. In fact, we have found out that if we wish to keep our microbial nursery in a healthy, flourishing condition, we must carefully banish all sources of light from our cultivations, and that a dark cupboard is one of the essential requisites of a bacteriological laboratory.