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.

One of the most interesting methods of extension of infection, showing on what small circumstances infection may depend, is seen in the case of the hookworm disease, which causes such devastation in the Southern States.  The organism which produces the disease, the Uncinaria, belongs to the more highly developed parasites, and is a small round worm one-third of an inch long.  The worms which inhabit the intestines have a sharp biting mouth by which they fasten themselves to the mucous membrane and devour the blood.  The most prominent symptom of the disease is anaemia, or loss of blood, due not only to the direct eating of the parasite, but to bleeding from the small wounds caused by its bite.  Large numbers of eggs are produced by the parasite which are passed out with the feces, which becomes the only infectious material.  In a city provided with water-closets and a system of sewerage there would be no means of extension of infection.  The eggs in the feces in conditions of warmth and moisture develop into small crawling larvae which can penetrate the skin, producing inflammation of this, known in the region as the ground itch.  The larvae enter the circulation and are carried to the lungs, where they perforate the capillaries and reach the inner surface; from this they pass along the windpipe, and then by way of the gullet and stomach reach their habitat, the small intestine.  Unfortunately, the habits and poverty of the people in every way facilitate the extension of the infection.  There is no proper disposal of the feces, few of the houses