about diflferences in health or sickness. Some parts of our population keep 

 well because they manage according to our best knowledge and make use of 

 expert knowledge and skills when there is need. Others are kept well by 

 being looked after by competent persons — as inmates of certain institutions. 

 But other parts of the population just drift along, and these consistently fur- 

 nish an excessive share of the ailing and the sick and the premature deaths. 



How Do Other Organisms Influence Our Health? 



Invaders^ The germ theory of disease, with which we commonly asso- 

 ciate the name of Louis Pasteur (see page 444), is really several hundred years 

 old. During the Middle Ages most physicians and scientists suspected that 

 plagues were due to "germs" carried from sick persons. But it was impossible 

 to prove the existence of these objects, because they are so small. The very 

 name malaria reveals the common understanding of the sources or causes of 

 disease. Everybody knew that it was the "bad air" — especially bad night air 

 — that brought on the fever and ague. People continued to speak of what 

 passed between one person and another as "vapors" or "miasmas" — and to 

 think of them as "spirits". 



Pasteur, who was not a physician, but a chemist, had discovered minute 

 objects as always present in the fermentation of wine and milk, and present in 

 sick silkworms. But he had not succeeded in proving beyond doubt that a 

 particular species of microbe was an essential factor in a particular disease. 



The first actual proof, or test, of Pasteur's germ theory was made by a Ger- 

 man physician, Robert Koch (1843-1910), working with an epidemic disease 

 of cattle — spleen fever, or anthrax. This proof consists of three distinct steps: 



1. Finding the specific bacteria or other suspected parasites always present 

 in every organism showing the symptoms of the disease; 



2. Isolating and multiplying the specific parasite in a pure growth outside 

 the body of the host, usually in a sterilized preparation of special food; 



3. Inducing the same disease in a healthy organism by inoculating it with 

 material from the pure culture. 



Bacteria of one kind or another will grow wherever there is organic mat- 

 ter, moisture, and a temperature not too low or too high. They are destroyed 

 by sunshine, by various chemicals, by X rays, and by the temperature of 

 boiling water. Many species endure prolonged boiling. The metabolism of 

 bacteria will be suspended when the temperature gets too low, but as a rule 

 microbes cannot be destroyed by freezing 



Some diseases are caused by plant parasites more complex than bacteria. 

 The skin disease known as ringworm is due to a moldlike fungus (see page 375) 

 and has nothing to do with worms. The irritation and damage are annoying 



iSee No. 3, p. 638. 

 612 



