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XXVIII] When cholera extends as an epidemic, its course is often singularly erratic. Some places, apparently in the direct line of advance, are passed over, to be attacked, perhaps, at a later period. Similarly, certain districts of a town may be spared, while other parts of the same town are ravaged by the disease. Local conditions favouring the presence of cholera.— On the whole, it may be said that low-lying districts, particularly those along river banks, are more subject to the disease than high and dry situations; and that overcrowding and unhygienic conditions generally conduce to its prevalence. The principal and special element, however, which determines the diffusion of cholera is, undoubtedly, the character of the water supply.

Cholera in the main a water-borne disease, entering by the stomach.— From time to time many theories of the cause and nature of cholera have been put forward, most of them very absurd and manifestly incorrect. Most of these have now been definitely abandoned in favour of the theory that the cause of cholera is a specific germ which, for the most part, is water-borne. The evidence in favour of this view may be regarded as being almost conclusive, although there Is still some room for doubting whether the germ itself has really been discovered. The earliest, and still one of the most telling pieces of evidence in favour of the water-borne theory of the diffusion of cholera, we owe to the late Dr. Snow. In August, 1854, cholera was epidemic in parts of London, notably in the neighbourhood of Golden Square, Soho. A child, after an illness of three or four days, died of the disease at 40, Broad Street, on the 2nd of September. The discharges from the patient had been thrown into a leaky cesspool which, as was subsequently discovered, drained into a well only three feet away. This well supplied the neighbourhood with drinking-water. On the night of the 31st of August cholera broke out among those who used the water of this particular well, very few escaping an attack. On the 2nd of September a lady died of cholera at Hampstead