We occasionally see epidemics of malaria in regions where it otherwise does not exist, or exists only in a mild form. At such times the malaria cases around the endemic foci also increase in number and severity.
The causes of malaria epidemics are usually to be found in widespread inundations, by which places otherwise free become adapted for the development of the malaria carrying mosquito. The malaria epidemic at Paris on the occasion of the construction of the boulevards is well known. In this ease it was undoubtedly the overturning of the earth that provided the conditions, viz.. small collections of water in which the anophelines can readily multiply and so transmit infection from the sick to the healthy. Yet there are also epidemics for which no reasons can be found. Epidemics usually limit themselves to one summer, but may recur several years after one another.
Europe was afflicted several times during the course of this century. The pandemic of 1806 spread over a large part of northern and northeastern Europe and lasted until 1812. An uncommonly large and severe pandemic, that spread almost over the whole earth, began in the year 1823 and lasted until 1827. Other epidemics occurred in the years 1845 to 1849, 1855 to 1860. 1S66 to 1872 (Hirsch).
Griesinger calls attention to the fact that malaria epidemics sometimes precede cholera epidemics, and refers to the occurrence of the two diseases after one another in India. Davidson assumes a close connection between the two infections. Dysentery, too. is frequently found endemic in the same places as malaria. Such observations as these it is also possible to harmonize with our present knowledge, for the water-the breeding place of mosquitos-is a factor which affects also the prevalence of such diseases as cholera and dysentery, but. of course, for a totally different reason.
Malaria has also been brought into relation with influenza, on account of sometimes following influenza epidemics. Hertz frequently observed malaria and typhus together, though he was unable to state anything about typhoid.