196 GENERAL BIOLOGY 



a mechanical obstruction of some kind or as a chemical poisoning. And 

 still again, there may be various poisonous substances formed by the 

 parasites themselves which will injure the immediately surrounding 

 tissues of the host only in the location of the parasites ; or, the poisons 

 may be soluble in the blood stream and in this way pass throughout the 

 entire body, injuring many regions. And there is still another way by 

 which injury is brought about by parasitical invaders. There are cer- 

 tain bacteria and protozoa which require considerable oxygen for their 

 life processes. The red blood corpuscles have become red by coming in 

 contact with the air in the lungs and absorbing oxygen which they then 

 distribute throughout the body. If the parasite, however, takes this 

 oxygen from the red blood cells, only carbon dioxide and carbon monox- 

 ide will remain. Carbon monoxide is "coal gas," a gas which often 

 asphyxiates men working in the coal mines. It therefore follows that 

 one may actually be "gassed" and die of this "gassing," should there be 

 parasites in the body which remove the much needed oxygen from the 

 red blood corpuscles. In such instances where oxygen is withdrawn, 

 death results almost immediately. 



Then there may be all manner of mixed infections, as they are 

 called. Just as it requires fire in order to cause powder to explode, so 

 there are certain chemical substances as well as living organisms which 

 by themselves do little harm or injury; but, when a second or third sub- 

 stance mixes with them, may prove quite injurious. Conversely, a single 

 substance may be quite injurious, such as either an acid or an alkali, 

 but when the two are mixed they neutralize each other and no active 

 injury is brought about. 



Everyone knows that no two people are exactly alike, in their ability 

 to resist disease and that one person may tolerate a much greater injury 

 than another without succumbing to it. Most of us have probably read 

 of the ancient king who, being afraid that an enemy might poison him, 

 took small doses of various poisons daily so that in due time he could 

 take great quantities without its having any injurious effect upon him. 

 That is, his toleration for this specific poison grew, and his body was 

 able to resist the usual injury caused by such poison. That is, his sys- 

 tem became insensible to specific poisons which were thus unable to 

 affect him injuriously because an immunity to these poisons had been 

 set up. 



Resistance, tolerance, and immunity may be classified in various 

 ways, such as racial, familial, and individual. As an example of race 

 immunity we have those groups of individuals living in the tropics who 

 do not succumb to the various tropical fevers that affect a stranger al- 

 most immediately. The classic example, however, is that of the Jews, 

 who having fought tuberculosis for thousands of years, are now more 

 immune than any other known race of mankind. The negroes and In- 



