DISTRIBUTION OF DISEASE GERMS 93 



The opening of the sanitary drainage canal in 1900, fur- 

 nishing Chicago a better means of sewage disposal and a cleaner 

 water supply, was an important factor in the reduction of 

 deaths from typhoid in Chicago. How much of the reduction 

 of the typhoid death rate is due to better milk and how much 

 to sewage disposal, better water, and other factors cannot be 

 definitely stated, but the facts of reduction as shown in the 

 following table are significant : 



CHICAGO TYPHOID DEATH RATE AFTER THE OPENING OF THE 

 DRAINAGE CANAL 



Typhoid deaths per Typhoid deaths per 



Year 100,000 population Year 100,000 population 



1900 19.8 1907 18.2 



1901 29.1 1908 . . . ..:'.. 15.8 



1902 34.5 1909 12.6 



1903 31.8 1910 13.7 



1904 ...... 19.6 1911 ...... 10.8 



1905 16.9 1912 7.5 



1906 18.5 1913 10.5 



103. Typhoid carried by flies. Disease bacteria may be 

 transmitted by many other agencies. For example, typhoid 

 bacteria may live in waste organic matter and may then be 

 carried by flies and deposited upon solid food or in milk or 

 water, and later gain entrance to the human body. The flies 

 are not causes of disease, they are merely the agents of trans- 

 mission. If no typhoid bacteria had been allowed to get to the 

 organic matter upon which the flies fed, the flies could not 

 have carried them. Since many people are so careless as to 

 allow distribution of disease bacteria to places from which 

 flies may carry them, it is important to study the habits of 

 the fly, with the hope of preventing further distribution of 

 bacteria (fig. 50). 



104. The life history of house flies. An individual house fly 

 lays about 120 eggs. They are usually laid in horse manure, 

 but may be placed in any animal or plant refuse. The eggs 



