524 TEXTBOOK OF BOTANY 



many years the water-borne organism causing typhoid fever took a 

 large annual toll in human lives. Cities are now practically free of this 

 disease; most typhoid cases originate from polluted well water in rural 

 communities. To eliminate the contamination of surface wells requires 

 merely careful location and construction of the wells with reference 

 to surface drainage which often contains large numbers of bacteria. 



Sanitary practices, such as quarantine, disinfection, admitting direct 

 sunlight into living rooms, cleanliness, cooking foods, pasteurizing milk, 

 and keeping foods in refrigerators, are all related to the elimination or 

 the reduction of the number and kinds of bacteria with which we come 

 into contact. 



Bacteria and disease. While most bacteria are not causes of disease, 

 some are, and these are known as pathogenic bacteria. The invasion of 

 the bodv tissues bv bacteria is known as infection. The severitv of the 

 disease depends upon the ability of the organisms to invade the tissues, 

 multiply rapidly, and cause injurv. The injurv to the bodv is due to the 

 destruction of tissues and the formation by the bacteria of certain poison- 

 ous substances called toxins. When the bodv is invaded, antitoxins, or 

 antibodies, are formed that neutralize the effects of the toxins either by 

 combining with them chemically, or otherwise rendering the cells im- 

 mune. In this way the body is protected until the bacteria are destroyed 

 by the colorless blood cells ( leucocvtes ) , or until the bacteria are made 

 harmless bv other means. 



The term immunity denotes the qualities of a plant or animal by 

 which the invasion or growth of pathogenic organisms is prevented, or 

 their products are rendered hamiless. A plant or animal is susceptible to 

 an infectious disease when such qualities are absent, or inadequately 

 developed. Not all persons are equallv susceptible to certain diseases, 

 and susceptibility may vary in the same individual from time to time. 

 A person is usuallv immune to a disease if his blood contains the 

 corresponding antibodv, or is able to produce it. Some of the more 

 common bacterial diseases of the human bodv are tuberculosis, pneu- 

 monia, diphtheria, tvphoid fever, and tetanus. 



Diseases of plants. Although most of the diseases of plants are caused 

 bv fungi, the destruction of plants by bacteria is often sufficient to 

 present a serious economic problem. Bacteria may be present in the 

 healthy tissues of some plants. They mav, however, penetrate the host 

 through wounds or natural openings and then spread throughout the 

 plant or remain localized within certain tissues. They are carried from 



