CHAPTER XIV 



THE PROTECTION OF HEALTH 



I. INFECTION AND IMMUNITY 



THE application of recently acquired knowledge in biological 

 research to the control and prevention of disease forms one of 

 the most interesting chapters in the history of the struggle for 

 human health. The results indicate clearly that the control 

 of disease depends upon the following facts : 



289. Classes of disease. Diseases are of two general 

 classes, the infectious and the non-infectious. Infectious dis- 

 eases are always caused by 

 living microorganisms. These 

 microorganisms are usually 

 described as pathogenic to 

 distinguish them from the 

 harmless and beneficial va- 

 rieties. 



Infectious diseases. The 

 microorganisms are of two 

 kinds : animal parasites, or 

 protozoa, and plant parasites, 

 as molds, yeasts, and bacteria. Protozoa (Fig. 172) are known 

 to be the cause of such diseases as malaria, yellow fever, rabies 

 or hydrophobia, amoebic or chronic dysentery, and possibly of 

 smallpox and scarlet fever. Bacteria (Fig. 173) are the causa- 

 tive organisms in such diseases as typhoid fever, tuberculosis, 

 diphtheria, pneumonia, influenza or grippe, colds, whooping 

 cough, cholera, tetanus (lockjaw), leprosy, rheumatism, bubonic 

 plague, certain forms of diarrhea and infantile paralysis. 



588 



FIG. 172. Two types of disease-caus- 

 ing protozoa : a, trypanosome, the cause 

 of sleeping sickness ; b, spirichasta, 

 the cause of relapsing fever, showing 

 manner of reproduction by lengthwise 

 division. 



