PLANTS WITHOUT CHLOROPHYLL 157 



which takes such a toll of death each summer, may be prevented 

 by pasteurizing the milk before using, so as to kill the harmful 

 bacteria. Other diseases, as malaria, yellow fever, sleeping sick- 

 ness, and probably smallpox, scarlet fever, and measles, are due 

 to the attack of one-celled animal parasites. Of these we shall 

 learn later in Chapter XV. 



Immunity. — It has been found that after an attack of a germ 

 disease the body will not soon be again attacked by the same 

 disease. This immunity, of which we will learn more later, seems 

 to be due to a manufacture in the blood of substances which 

 fight the bacteria or their poisons. If a person keeps his body 

 in good physical condition and lives carefully, he will do much 

 toward acquiring this natural immunity. 



Acquired Immunity. — Modern medicine has discovered means 

 of protecting the body from some contagious diseases. Vaccina- 

 tion as protection against smallpox, the use of antitoxins (of which 

 more later) against diphtheria, and inoculation against typhoid 

 are all ways in which we may be protected against diseases. 



Methods of fighting Germ Diseases. — As we have seen, dis- 

 eases produced by bacteria may be caused by the bacteria being 

 directly transferred from one person to another, or the disease 

 may obtain a foothold in the body from food, water, or by taking 

 them into the blood through a cut or a wound or a body opening. 



It is evident that as individuals we may each do something to 

 prevent the spread of germ diseases, especially in our homes. We 

 may keep our bodies, especially our hands and faces, clean. Sweep- 

 ing and dusting may be done with damp cloths so as not to raise a 

 dust ; our milk and water, when from a suspicious supply, may be 

 sterilized or pasteurized. Wounds through which bacteria might 

 obtain foothold in the body should be washed with some antiseptic 

 such as carbolic acid (1 part to 25 water), which kills the germs. 

 In a later chapter we shall learn more of how we may cooperate 

 with the authorities to combat disease and make our city or town 

 a better place in which to live.^ 



1 Teachers may take up parts or all of Chapter XXIV at this point. I have 

 found it advisable to repeat much of the work on bacteria after the students have 

 taken up the study of the human organism. 



