22 



Cofistaiit Care Is Needed for Health unit vi 



of every 100,000 people died of tubercu- 

 losis; in 1930 the total death rate of all 

 disease was only very slightly more than 

 that. Figure 292 is a chart of the death 

 rate from tuberculosis in New York City 

 and in the whole United States. This 

 chart should be studied carefully. See 

 Exercise i. While tuberculosis can af- 

 fect any organ, the most common form 

 is lung or pulmonary tuberculosis. Since 

 the germs of pulmonary tuberculosis are 

 easily spread by coughing, and since in- 

 fected people are generally not confined 

 to their beds, the disease can spread 

 rapidly. Koch's discovery of the germ 

 was of great importance. When the 

 cause of the disease became known, 

 measures could be taken to stop its 

 spread. There is now a widespread cam- 

 paign to teach people how to prevent 

 tuberculosis. 



Doctors can now detect the disease in 

 the early stages by chest x-rays. When 

 people have these x-rays taken fre- 

 quently as part of a regular medical 

 checkup, the death rate from tubercu- 

 losis will be reduced still further because 

 in its early stages tuberculosis is rela- 

 tively easy to cure. 



Koch's four postulates. Koch always 

 found a certain type of bacterium in ani- 

 mals suffering from tuberculosis, but this 

 was not enough to convince him that the 

 bacterium caused the disease; perhaps it 

 merely accompanied the disease. He felt 

 that four steps must be successfully taken 

 before one could say that a disease is 

 caused by a specific germ. The four steps 

 are often spoken of as Koch's postulates. 

 See the next column. Dcj the four postu- 

 lates provide evidence for the si^erm 

 theory of disease? See Exercises 2 and 3. 



Koch's Postulates 

 To prove that a disease is caused by a 

 germ: 



1. The germ must be found in every or- 

 ganism that has the disease. 



2. The germ must be grown in pure cul- 

 ture so that it can be definitely iden- 

 tified. 



3. When the germs grown in the labora- 

 tory are injected into healthy animals, 

 these animals must show all symptoms 

 of the disease. 



4. The germ must be recovered from the 

 sick animals and must be identified as 

 the original germ. 



Koch did all this before announcing- 

 that he had found the cause of tubercu- 

 losis. As you read further, however, you 

 will see that it is not always possible to 

 take these four steps. In fact, we must 

 even modify our understanding of the 

 germ theory of disease to some extent 

 because of recent discoveries. 



The early contributions of Louis 

 Pasteur. As a rule a scientist can work 

 out every step of a problem only with 

 direct or indirect help from others. Had 

 Koch failed to read the scientific journals 

 of his day, had other bacteriologists not 

 taken an interest in his work and dis- 

 cussed it with him, the germ theory of 

 disease could not have been established. 

 There were many men in many coun- 

 tries working along these same lines at 

 this time. Among them was Louis Pas- 

 teur ( 1 822-1895), "^^'lio shares with Koch 

 the credit for giving to the world the 

 germ theory of disease. 



Louis Pasteur discovered that specific 

 microorganisms changed grape juice to 



