When the writer first went to India some quarter of a century ago, there was still a tendency to invest this disease and its propagation with certain mysterious attributes, and a certain pompous obscurantist who was then at the head of affairs invented the awe-inspiring term of “pandemic waves” to account for, or rather cover, ignorance as to its method of spread; nor was it altogether safe for his subordinates to record facts that appeared to indicate a more common-sense explanation. But even then, the theory that the disease was usually conveyed from man to man by infected water, was held practically by all whose opinions were worth having. Still we were very much in the dark as to the methods whereby it gained access to water, and had no means of distinguishing infected from harmless water; and our measures of prevention, being thus based on guess work, were uncertain and often ineffective. At the present day, thanks in the main to the labours of Koch and Hankin, there is no disease about which our knowledge is more definite. In dealing with semicivilised communities it is still, it is true, quite impossible to prevent or foresee outbreaks of the disease, but when it occurs our knowledge now enables us to bring an epidemic to a speedy termination, always provided we are permitted to do so by the population; and personal prophylaxis may almost be said to amount to security. We now know that the germs of cholera can exist only in the human organisation, but that they are capable of living and also _multiplying_ in water. In very impure water, they cannot long survive, as they soon get crowded out of existence, in such situations, by putrefactive and other germs that are most at home under such conditions. For their _multiplication_ the presence of a certain amount of otherwise harmless organic and mineral matter is of course necessary, but if introduced into water containing this, they can survive some time in the purest natural waters, though they ultimately die out in such situations. Owing to these habits of life on the part of the germ, it follows that the taste and appearance of water are absolutely valueless as regards its safety, and that the _chemical_ examination of water is an equally futile test. The disease is carried about from place to place by infected human beings; but in ninety-nine cases out of a hundred is conveyed from man to man not directly, but indirectly through the agency of drinking water. In the hundredth case the germs may be carried in food, and of course milk, being often intentionally or accidentally mixed with water, is a frequent vehicle.