If a mosquito of the species _anopheles_ bites the affected person, it obtains a large amount of blood which contains many parasites. Within the mosquito the parasite undergoes a further development into male and female sexual forms, which may also form in the blood, termed respectively _microgametocyte_ and _macrogamete_. From the microgametocyte small flagellate bodies, the male sexual elements _microgametes_ or _spermatozoa_, develop and fertilize the _macrogametes_; after fertilization this develops into a large body, the _ooecyst_ which is attached to the wall of the stomach of the mosquito. Within the ooecyst, innumerable small bodies, the sporozoites, develop, make their way into the salivary glands and are injected into the individual who becomes the prey of the mosquito, and again the cycle of development begins. The presence of the parasite within the mosquito does not constitute a disease. So far as can be determined, life goes on in the usual way, and its duration in the insect is not shortened. The nature of the parasite which produces yellow fever is unknown, for it belongs to the filterable viruses; the infectious material, however, has been shown by inoculation to exist in the blood, and the disease is transmitted by a mosquito of another species, the _stegomyia_. The development cycle within this takes a period of twelve days, which time must elapse after the mosquito has bitten before it can transmit the disease. Here again the mutual interdependence of knowledge is shown. Nothing could have seemed less useful than the study of mosquitoes, the differentiation of the different species, their mode of life, etc., and yet without this knowledge discoveries so beneficial and of such far-reaching importance to the whole human race as that of the cause and mode of transmission of malaria and yellow fever would have been impossible; for it could easily have been shown that the ordinary _culex_ mosquito played no role. The role which insects may play in the transmission of disease was first shown by Theobald Smith in this country, in the transmission by a tick of the disease of cattle known as Texas fever. The infecting organism _pyrosoma bigenimum_ is a tiny pear-shaped parasite of the red corpuscles. Smith's investigations on the disease, published in 1893, is one of the classics in medicine, and one of the few examples of an investigation which has not been changed or added to by further work.