IMMUNITY 271 



culosis germ attacks only guinea pigs, monkeys, and man. Small- 

 pox and cowpox are probably caused by the same organism. 

 Plague attacks rats, ground squirrels, mice, and guinea pigs, as 

 well as man. A long series of laboratory tests show that most germs 

 that cause illness in man develop ordinarily in man only, while a 

 few diseases, Hke anthrax and glanders, are primarily diseases of 

 certain animals but may attack man. 



Immunity may be modified by External Conditions. — A certain 

 amount of immunity is evidently natural to individuals, races, or 

 species, but there is much variation, as we have seen, even among 

 individuals of the same family. Resistance to disease also is 

 modified by the condition of the individual exposed. Overworked, 

 tired, and ''run-down" persons are much more likely to take germ 

 diseases than those who are in good physical trim. Resistance to 

 disease may also be weakened by the use of drugs and alcohol 

 as shown by the susceptibility of heavy drinkers to pneumonia. 



Acquired Immunity. — It has been a matter of common knowl- 

 edge for centuries that persons who have infectious diseases do not 

 usually have them a second time. A Greek historian, describing a 

 visitation of plague in Athens, more than twenty centuries ago, noted 

 that those who had plague and recovered were safe from it there- 

 after. The Chinese, in order to make their children immune to 

 smallpox, gave them the disease in a mild form by placing in the 

 nose a little of the pus from one of the eruptions. And it was the 

 chance statement of a dairymaid in England when she said, '' I've 

 had cowpox and can't take smallpox " that led Edward Jenner 

 (page 407) to make his first experiments that have resulted in 

 almost stamping out smallpox through vaccination. And so to-day 

 when we think of acquired immunity obtained by this or that 

 antitoxin or antiserum or vaccine, we must remember those 

 pioneers, Jenner and Pasteur (page 408), who took the first steps 

 in controlling germs, and began the work which may result finally 

 in preventing most germ diseases. 



How Immunity is gained. — We have already seen that the 

 blood contains small amounts of various specific substances, known 

 collectively as antibodies (page 165). These help the cells of the 

 body combat harmful bacteria, the poisons or toxins which they 

 give out, and the poisonous "split proteins" which are thrown 



