A very striking and at first sight astonishing fact in regard to this subject is that there are a very large number and variety of microbes habitually present in the human digestive tract. There are so many different kinds—differing altogether from one another in their chemical action—which are present in greater or less abundance in this tract from one end to the other, that no one is at present able to say even approximately how many there are, nor to give anything like a complete account of their properties. The fact is that their isolation and study, and the definite determination of their properties, is not an easy job. Many workers are engaged on it, and it will be years before the matter is threshed out. One most curious result of these studies is that a person may have the cholera-bacillus in his intestine—not growing with any activity, but still alive—and yet be perfectly well. He can, therefore, carry the cholera-bacillus from one locality to another and spread the disease, and yet be entirely devoid of suspicion, free himself from disease, and certified as healthy! The same is true of the bacillus of typhoid fever. Persons who have had typhoid fever have been shown to retain the typhoid-bacillus flourishing for as long as fourteen years afterwards in their intestine, without any ill effects to themselves, and to have been the constant source of infection and disease to those living in the same house with them by spreading the bacillus. The classical case of this is that of a woman who carried on a baker’s business at Strasburg. Infection by and protection from microbes is by no means so simple a thing as it is sometimes represented to be. Now that we are quite sure as to Koch’s comma-bacillus, or spirillum, being the definite poison-producing agent causing Indian cholera, it is comparatively easy to understand its mode of dispersal and infection, and consequently how to avoid its attack. It is cultivated in the laboratories devoted to the study of such matters—kept in confinement, so to speak, for ten years and more—and its properties and conditions of life are well known. For instance, it is destroyed by “dryness,” hence it cannot be carried in a living infective state as “dust” in the air. It is also destroyed by exposure to a heat a good deal below that of boiling water, so that water itself can be freed from it by boiling, and food dipped in boiling (or nearly boiling) water, or heated on a metal tray beneath which a spirit or gas flame is burning, can be rendered safe just before it is swallowed, even when cholera is rife in the neighbourhood. Ordinary lime is a great destroyer of the bacillus, and can be used on a large scale to abolish it in refuse.