If the seat of disease be in the lungs, the throat or the mouth, the sputum forms the medium of extension, which can take place in many ways.

The sputum may become dried, forms part of the dust and the organisms enter with the inspired air.

The organisms which cause most of the diseases in which the sputum becomes infectious are quickly destroyed by conditions in the open, such as the sunlight and drying; street dust does not play so prominent a part in extension as is generally supposed.

Organisms find much more favorable conditions within houses.

It is now generally recognized that infection with tuberculosis does not take place in the open, but in houses in which the bacilli on being discharged are not destroyed.
The hands, the clothing and surroundings even with the exercise of the greatest care may become soiled with the saliva.
It has been shown that in coughing and speaking very fine particles of spray are formed by the intermingling of air and saliva, which may be projected a considerable distance and remain floating in the air for some time.