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.