We have learned in our study of Household Bacteriology that nearly all diseases, especially those coming under the head of infectious and contagious, are caused by certain species of bacteria.

If we would be immune from these diseases, then we must do everything in our power to exclude these germs. Cleanliness, plenty of sunlight and fresh air, are the first requisites for their exclusion; and, when disease has entered, proper isolation and disinfection to prevent their spread.

By disinfection we mean destruction of the bacteria by use of certain chemicals or heat. Heat, when it can be used, is always the surest and quickest method. The rules for disinfection, or sterilizing by heat, will be given under the head of "Surgical Operations at Home."

The disinfectants most commonly used in illness are bichloride of mercury, 1-1ooo, for the hands and utensils, and carbolic acid, 1-20, for the clothes, instruments, etc. Bichloride is the stronger disinfectant, but as it discolors clothes and instruments it should not be used for them.

Disinfection