o 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.

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 conveye