months, are most numerous during the second and third year, and become rare after five years' residence. This is exactly what might be expected to happen with any localized infectious disease. The relatively small number of cases within the first six months may be explained by non-exposure to the causative agents of the disease; indeed, many arrive in the endemic district at the end of the epidemic season. The progressive prevalence of cases during the first three years finds an obvious explanation in the accumulating chances of infection; the decrease in later years may be partly accounted for by the progressive diminution in the number of the older residents and by the weeding out of the most susceptible. Daniels states that the early cases of blackwater fever those under one year's residence are mainly (14 out of 21) in persons resident at or below the 1,500 feet level; and the majority (10 out of 14) of cases in persons having four years' residence are in persons residing principally in the highlands.
Etiology.— There are three theories as to the etiology of blackwater fever 1, the "malaria" theory; 2, the quinine theory; 3, the specific theory.
1. The malaria theory.— The prevalence of blackwater fever in malarious regions, the great frequency of its occurrence in persons who had previously suffered from one or other of the malarial fevers, the finding of malaria parasites and hæmozoin in the blood and organs, and the large mononuclear leucocytosis of hæmoglobin uric cases led to the belief that blackwater fever was an unusually severe form of "malaria." Against this theory is the fact that although blackwater fever is certainly co-endemic with one or other form of malaria in several regions, it is not so in all regions. It has its own peculiar distribution, and is absent or very rare in many places in which the various intermittent fevers are especially rife. It is exceedingly common amongst the few Europeans who live in tropical Africa; it is practically unknown amongst the many thousands of Englishmen who live in the fever haunts of India and elsewhere.
