Transmission by air plays but little part in the extension of infection. In such a disease as smallpox, where the localization is on the surface of the body, the organisms are contained in or on the thin epithelial scales which are constantly given off. These are light, and may remain floating in the air and carried by air currents just as is the pollen of plants. There seem to have been cases of smallpox where other modes of more direct transmission could be excluded and in which the organisms were carried in the air over a considerable space. All sorts of intermediate objects, both living and inanimate, such as persons, domestic animals, toys, books, money, etc., can serve as conveyors of infection. Insects play a most important part in the transmission of disease, and in certain cases, as when a disease is localized in the blood, this is the only means of transmission. There are three ways in which the insect plays the role of conveyor. 1. The insect may play a purely passive part in that its exterior surface becomes contaminated with the discharges of the sick person, and in this way the organisms of disease may be conveyed to articles of food, etc. The ordinary house fly conveys in this way the organisms of typhoid and dysentery. Flies seek the discharges not only for food, but for the purpose of depositing their eggs, and the hairy and irregular surface of their feet facilitates contamination and conveyance. When flies eat such discharges the organisms may pass through the alimentary canal unchanged and be deposited with their feces; they also often vomit or regurgitate food, and in this way also contaminate objects. Flies very greedily devour the sputum of tuberculous patients, and the tubercle bacilli contained in this pass through them unchanged and are deposited in their feces. [Illustration: FIG. 19.--TRYPANOSOMES FROM BIRDS. All the trypanosomes are very much alike. They contain a nucleus represented by the dark area in the centre and a fur-like membrane terminating in a long whip-like flagellum. They have the power of very active motion within the blood.]