162 



HUMAN BIOLOGY 



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 

 scrum of the blood also develops antitoxins which neutral- 

 ize the toxins formed in disease. Not only can the white 

 corpuscles and serum kill bacteria, but most of the secre- 

 tions 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 eat- 

 ing 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 anti- 

 toxin, is developed which cures the first attack and remains 

 to protect the body in future. 



The periods of quarantine or isolation for several com- 

 mon 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- 



