148 THE CONTROL OF LIFE 



disease requires, if it is to express itself, is just the 

 average nurture of everyday life. It is probable that 

 gout, albuminuria, and some forms of diabetes and 

 nervous instability are examples of constitutional 

 disease. The biological tactics are to try by counter- 

 active nurture to save the body from having to fight 

 a battle with two fronts — which is seldom hopeful. 



The second class of diseases may be called microbic. 

 These diseases are due to the invasion of the body 

 by injurious organisms — usually of microscopic size. 

 Some of the microbes incline to the plant-like mode 

 of life, and most of these are called bacteria. Thus 

 bubonic plague (the Black Death), cholera, tuber- 

 culosis, tetanus, typhoid fever, and many other dis- 

 eases are due to bacteria. Other microbes incline to 

 the animal mode of life or are undoubted animals. 

 Thus sleeping-sickness, malaria, and syphilis are due 

 to microscopic animals or Protozoa. Some larger ani- 

 mal parasites sometimes cause very serious human 

 diseases. Thus about every third native of Lower 

 Egypt suffers from a very serious disease called Bil- 

 harziasis, which is due to a worm allied to the liver- 

 fluke of sheep. One of the great life-savers of the 

 period of the Great War was Dr. Leiper, who dis- 

 covered the life-history of the Bilharzia-worm, and has 

 shown how its invasion of the human body may be 

 frustrated. In connection with these microbic and 

 parasitic diseases, we must understand that conquering 

 them depends in many cases on discovering the life- 

 history of the intruder. A remarkable fact which has 



