PROBLEM A What Have Scientists Learned About 



Conquering Some Common Diseases? 



The germ theory of disease. You are so 



accustomed to the idea that germs cause 



o 



certain diseases that it is difficult for you 

 to reahze that man has not always 

 known this. At the time when the early 

 settlers came to this country, medical 

 students were still being taught that dis- 

 ease was caused by some unusual posi- 

 tion of the planets! In the days of George 

 Washington men had no suspicion that 

 diphtheria and pneumonia and other dis- 

 eases were caused by living microorgan- 

 isms. It was a century ago, within the 

 memory of your great-grandfather, that 

 an Italian scientist took the first step to- 

 ward the discovery of the germ theory 



of disease — the theory that certain dis- 

 eases, those now known as injectious dis- 

 eases, are caused by definite microorgan- 

 isms. He noticed that all silkwomis 

 which were afl^ected by a disease com- 

 mon at the time were hosts to a very 

 small parasitic fungus. There were no 

 exceptions, so he concluded that the 

 fungus was the cause of the disease. 



As time went on other discoveries of 

 this kind were made; for example, some 

 unimportant skin diseases of man were 

 shown to be caused by fungi of one kind 

 or another. 



About 1875, suspecting the part played 

 by bacteria in disease, Koch directed his 



Fig. 291 One Africivi tribe 

 treats all aches and pains by 

 bleeding the patient. The 

 ^\ioctor" makes a cut over 

 the affected part. He then 

 places a goat horn over the 

 cut and sucks the air front 

 the horn. These people 

 know nothing of the germ 

 tl.^eory of disease. When 

 ivas the germ theory estab- 

 lished? (CHICAGO NATURAL 

 HISTORY museum) 



