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278 manifestations of the infection and relapses may occur at any season and in any place.

Epidemiology.— Blackwater fever at times assumes an epidemic form. It may not be seen for years in a district, and then numbers of cases may occur within a short time. Very often, as is the case in yellow fever, the magnitude of an " epidemic" may depend on the number of susceptible persons, possibly new arrivals, within the endemic region. It broke out amongst the labourers employed in making the canal through the Isthmus of Corinth; it attacked the Chinese labourers on the Congo railway; and in 1885, according to Dr. Wenyon, of Fatshan, China, " it ravaged like a plague the Chinese army on the Tonquin border of Kwangsi." In collective dwellings, such as barracks, hospitals, schools, it may attack several persons at the same time. In 1885 it broke out in a prison in Castiades, Sardinia, attacking 24 out of 800 convicts. Sometimes several cases may occur at intervals in the same house; such houses are known in British Central Africa as "blackwater fever houses."* Predisposing causes.— Individuals of all ages and both sexes are liable to blackwater fever, but they are not equally subject to it. It more commonly affects men about the middle period of life, obviously on account of their greater exposure either to its causative agent or to circumstances provocative of an attack.

At one time race was considered an important factor. In Africa, Europeans, Indians, and Chinese are attacked in great numbers, whilst the natives are said to enjoy an immunity. The immunity of the natives is probably not a racial immunity; more likely it is the immunity conferred by infection during childhood. In fact, negroes living in places