218 



LIFE ON THE EARTH 



is extracted by the salt. Lactic-acid bacteria that live 

 and grow in the juice from the cabbage attack the 

 cabbage and change it into sauerkraut. 



Bacteria are useful in agriculture. The importance 

 of bacteria in maintaining soil fertility was discussed 

 in the unit on rocks and soils. Growing plants re- 

 move important chemical elements from the soil. Bac- 

 teria of decay attack plant and animal remains and 

 decompose the complex substances of their bodies 

 into simpler substances. These simpler substances 

 are used over again by other plants. Were it not for 

 the action of these helpful bacteria, nitrogen and other 

 necessary elements would soon be gone from the soil, 

 and all plant life on the earth would cease. 



What is the relation of bacteria to disease? When 

 certain bacteria live and grow in the human body, 

 they give off poisonous substances called toxins 

 (poisons). These poisons are carried by the blood to 

 all parts of the body, and they .cause illness by in- 

 terfering with the normal activities of the body. To 

 counteract the toxins, the body produces substances 

 called antitoxins. If the body is able to produce anti- 

 toxins faster than the bacteria produce toxins, the 

 disease is checked. If not enough antitoxins are pro- 

 duced, death may result. 



How do persons become immune to a disease? Not 

 all persons are equally susceptible to all bacterial 

 diseases. If disease-producing bacteria .enter an in- 

 dividual's body, but he is able to resist the disease, 

 he is said to be immune to that disease. An individual 

 that possesses immunity to a disease from birth has 

 natural immunity to that disease. Members of the same 

 race of people differ in their ability to resist diseases. 

 Some children, for example, are naturally immune to 

 diphtheria, whereas others have no resistance to it. 

 Also there are racial differences. The white race is 

 fairly resistant to tuberculosis, whereas Eskimos, Ne- 

 groes, and Indians are highly susceptible to this dis- 

 ease. Measles is often fatal to natives of certain 

 South Sea islands, but it is a relatively mild disease 

 with white people. 



Immunity developed during the lifetime of a per- 

 son is called acquired immunity. Immunity may be ac- 

 quired in three different ways. 



By having the disease. Persons who have had measles, 

 smallpox, diphtheria, scarlet fever, mumps, or whoop- 

 ing cough do not usually have the same disease again. 

 Unfortunately not all diseases produce immunity in 

 those who recover from them. Influenza and pneu- 

 monia may return many times. Immunity develops 

 during the course of these diseases, and the patient 

 recovers from them, but the immunity does not last 

 long. 



By vaccination. Vaccination consists of injecting 

 dead or weakened germs into the body. The toxins 

 that these germs produce stimulate the body to pro- 



duce its own antitoxins. Dead bacteria are used in 

 vaccination against typhoid fever. Weakened germs 

 from a cow are used in smallpox vaccination. 



The history of smallpox vaccination is interesting. 

 Vaccination was practiced in the Orient centuries 

 ago. The people would obtain pus or scabs from a 

 smallpox patient and put 

 it into a cut or scratch of a 

 well person. This caused 

 the disease and developed 

 immunity in those who re- 

 covered. This method of 

 vaccination was intro- 

 duced into England from 

 Constantinople during the 

 eighteenth century. 



It has been observed for 

 a long time that persons 

 who had contracted cow- 

 pox, a disease of cows, 

 were immune to smallpox. 

 Edward Jenner (1749- 

 1823), an English physi- 

 cian, proved by a series of 

 brilliant experiments that 

 cowpox is a form of small- 

 pox and that having cow- 

 pox makes one immune to 

 smallpox. After Jenner 

 made known his ex- 

 periments to the world, 

 much opposition devel- 

 oped against his method 

 of vaccination. Newspa- 

 pers bitterly attacked the 

 method, but the success- 

 ful results obtained led 

 eventually to the adoption 

 of Jenner's vaccination for 

 the control of smallpox 

 and to the prevention by 

 law of the older and dan- 

 gerous method. Vaccina- 

 tion was introduced into 

 this country about 1800. 

 There are still a number 

 of states that do not have 

 compulsory vaccination. 



By injection of a toxin. 

 Immunity to diphtheria is 

 produced in children by 

 injecting a mixture of 

 toxin and antitoxin. The 



toxin stimulates the body to develop its own perma- 

 nent immunity, while the antitoxin lessens any ill 

 effects the toxin might otherwise produce. Immunity 



School of Medicine, Washington 

 University 



FIG. 346. DISEASE GERMS 



Top Tuberculosis 



Middle Diphtheria 



Bottom Typhoid 



