munication with other ports already infected. In India, although the problem is much more difficult to unravel, in certain instances the influence of human intercourse in diffusing the disease can be distinctly traced. Thus the extensive pilgrimages, so frequent in that country, are a fruitful source of its rapid spread. During these gatherings hundreds of thousands of human beings are collected together under highly insanitary conditions— as at the Hurdwar and Mecca pilgrimages. Cholera breaks out among the devotees, who, when they separate, carry the disease along with them as they proceed towards their homes, infecting the people of the places they pass through. Cholera never travels faster than a man can travel; but in modern times, owing to the increased speed of locomotion and the increased amount of travel, epidemics advance more rapidly and pursue a more erratic course than they did sixty years ago.
Isolation secures immunity.— In the case of isolated countries the absence of active and frequent intercourse with the outer world favours immunity, even during approximately pandemic extensions. Thus, though so near to the reputed home of cholera the Andaman Islands have never been visited by that disease. Similarly, Australia and New Zealand hitherto have enjoyed practical exemption. The same can be said of the Pacific Islands, the Cape of Good Hope, the West Coast of Africa, Orkney and Shetland, Iceland, the Faroe Islands, and many of the islands of the Atlantic.
Unequal diffusion in the endemic and epidemic areas.— Although cholera is always present in some part of the endemic area in Bengal, it is not equally diffused there, nor is it equally common at all seasons and every year. Thus, even within this area there are places which enjoy an absolute or a relative immunity, and there are seasons and years of special prevalence. It has also to be remarked that the season of immunity for one place may be the season for prevalence in another place, and vice versa. The same observations apply to the areas of epidemic extension.
