INFECTION AND IMMUNITY. 35 



tion of the poisons or toxins made by the bacteria. 

 These poisons may be extracellular or intracellular. 

 The extracellular toxins are thrown out of the bodies Toxins 



extra- and 



of the bacteria into the tissues or media in which they 

 are growing. The word toxin when used alone is 

 taken to mean an extracellular toxin. The intracel- 

 lular or endotoxins are retained within the bodies of 

 the bacteria and are set free only after their death or 

 dissolution. After absorption the bacterial toxins do 

 not affect all organs or tissues equally, but exhibit a 

 selective action, some attacking the red blood-cor- 

 puscles and dissolving them, others the tissues of the 

 brain and nervous system. 



One might think, from what has been said, that immunity 

 men and animals are wholly at the mercy of bacteria. 

 Fortunately this is not so, as all are endowed with 

 certain defensive powers that resist the injurious 

 action of bacteria and their poisons. This resistance 

 to disease is called immunity. 



Many of the diseases that are infectious in man 

 cannot be transmitted to animals and, conversely, some 

 of the infectious diseases of animals do not occur in 

 man. 



Among the races of men variations in the resist- 

 ance to disease is observed; for example, the negro 

 seems to possess a much greater resistance to infection 

 with yellow fever than the white man. In addition to 

 the variations in resistance among the races of man 

 there are also variations among individuals. The 

 conditions under which people live have much to do 



