finding in young children after death is small areas of inflammation in the lungs in and around the terminations of the air tubes.  The situation renders it evident that the organisms which caused the lesions entered the lung by the air tubes.  The mouth of the child is unclean and harbors numbers of the same sort of organisms as those causing the lung inflammation; but in the absence of such a mode of infection as is given by spray formation it is difficult to see how the extension from the mouth to the lungs could take place.  The weakened condition of the body in these cases favors the secondary infection.

If the disease be located in the intestines, as in typhoid fever and dysentery, the organisms are contained in the fecal discharges, and by means of these the infection is extended.  In typhoid fever, dysentery and cholera massive infections of the populace may take place from the contamination of a water supply and the disease be extended over an entire city.  One of the most striking instances of this mode of extension was in the epidemic of cholera in Hamburg in 1892.  There were two sources of water supply, one of which was infected, and the cases were distributed in the city in the track of the infected supply.  Many such instances have been seen in typhoid fever.  Certain articles of food, particularly milk, serve as sources of infection.  This is more apt to happen when the organism causing the infection grows easily outside of the body.  A few such organisms entering into the milk can multiply enormously in a few hours and increase the amount of infectious material.  In all these cases the sick individual remains a source of infection, for it is almost impossible to avoid some contamination of the body and the immediate surroundings with the organisms contained in the discharges.

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.