CHAPTER XX 



SANITATION 



421. Bacteria and Disease. We have already learned 

 (cf. 324) that bacteria are plants belonging to the class 

 of fungi, and that they cause many fermentations, the 

 decay of dead animals and plants, and diseases that are 

 "catching," or contagious. All disease germs are not, 

 however, bacteria; some of them are minute one-celled 

 animals (protozoa). Thus, malaria and yellow fever are 

 caused by protozoa. It is impossible for us to escape 

 from germs, for they are everywhere about us: on the 

 ground, in the air, and in our houses ; but, fortunately for 

 us, all germs do not cause disease. Most of them live 

 their own lives without interfering with ours. In the 

 outside air we should rarely meet with injurious germs 

 were it not that they escape from the bodies of animals 

 and of persons who are sick. So the germs we usually 

 have to fear are those that are carried, in food or by con- 

 tact, from animals to persons, or from one person to 

 another. 



The "germ theory" of contagious diseases did not gain immediate 

 acceptance when it was first stated, for men could not believe that 

 organisms so small could do so much mischief. But the patient labor 

 of one investigator after another proved that the theory is correct. 

 We now know that germs are responsible for smallpox, measles, 

 diphtheria, scarlet fever, typhoid fever, tuberculosis, influenza (la 

 grippe), malaria, yellow fever, pneumonia, lockjaw (tetanus), mumps, 

 cholera, leprosy, the plague, etc., etc. It takes a definite kind of germ 



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