STUDIES OF SPORE-PLANTS 295 



tion ; and also it is a rule that one is not likely to have the 

 same diseases a second time. Furthermore, adults are not 

 likely to have the diseases which commonly affect children. 

 This lack of susceptibility to diseases is known as immunity. 

 Immunity which is present in human or animal individuals 

 who are not susceptible to a certain disease is known as natural 

 immunity, while that which follows an attack of a disease 

 is said to be acquired. 



Natural immunity is much more common than is suscep- 

 tibility to germ diseases. Probably the bacteria cause 

 disease in only a small percentage of the individuals which 

 they enter. It is known that germs of pneumonia and other 

 diseases are often present in persons who show no signs of 

 disease. The reason why we do not develop disease every 

 time a pathogenic organism enters our bodies is due to 

 (a) destruction of bacteria by the white cells of the blood 

 and lymph, (6) killing of bacteria by opposing soluble sub- 

 stances in the blood, (c) prevention of growth of bacteria by 

 antiseptic conditions in the body, and (d) counteracting of 

 toxins of the bacteria by antitoxins secreted by the cells of 

 our bodies. The relative value of these four methods of 

 protection varies with health and with individuals. In 

 general, all four are most efficient when there is good health, 

 and hence hygiene by improving general health helps the 

 human body in opposing pathogenic organisms. A good 

 illustration of this is the fact that building up the general 

 health is so all-important in the cure, as well as in the pre- 

 vention, of consumption or tuberculosis. 



Vaccination is an example of acquired immunity produced 

 without an attack of smallpox, but by substitution of a simi- 

 lar and harmless disease known as cow-pox. ,In fact, cow- 

 pox seems to be smallpox which has been weakened by devel- 

 oping in cows ; and so when vaccine matter is taken from the 

 pustules on cows and rubbed into a cut or scratch on the 

 human skin, the result is a mild development of cow-pox. 



