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 Further Means of Protection against Disease Germs.—The best protection is physical vigor. There are certain substances called opsonins which exist in the plasma of the blood of disease-resisting persons; these opsonins give the white corpuscles the power to devour disease germs. The serum of the blood also develops antitoxins which neutralize the toxins formed in disease. Not only can the white corpuscles and serum kill bacteria, but most of the secretions of the healthy body (gastric juice, nasal secretions, etc.) are bacteria-killing as well. Persons in a low state of health most readily succumb to disease. Excess in eating may lessen the germicidal power of gastric juice and inactivity that of the lymph. The same germ disease does not usually attack the same person twice, as the body becomes immune; that is, an opsonin, or an antitoxin, is developed which cures the first attack and remains to protect the body in future.

The periods of quarantine or isolation for several common germ diseases are given in the following table:—

Water Supply.—Bacteria are more abundant in flowing streams than in water standing in lakes or reservoirs (con-