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 rôle 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