week, 31 after two weeks, 5 after three weeks, and none after 

 four weeks. The danger against which we must guard is the 

 rather direct transfer of infectious material from one person 

 to another (or in a few instances from one of the higher 

 animals, such as the cow, to man). 



The source of disease germs is then the human (or in a few 

 cases, the animal) body. An important „. ~ 



part in the spread of communicable disease . f 

 is played by early cases (not yet displaying . 



any characteristic symptoms of illness) and 

 by "carriers," persons who have recovered from their illness 

 or may never have themselves suffered from a particular dis- 

 ease at all, and yet are cultivating in their bodies and spread- 

 ing to others the germs which are capable of causing the 

 malady in question. An outbreak of over three hundred 

 cases of typhoid in New York City was caused by a milkman, 

 a typhoid carrier, who had had typhoid in Michigan forty- 

 six years before and had been cultivating the germs in his 

 body ever since. One or two out of a hundred well persons 

 in a given community may be cultivating the germ of diph- 

 theria in their noses or throats, and one or two out of a 

 thousand may be cultivating the germ of typhoid fever in 

 gall bladder or intestines. In such diseases as pneumonia 

 and epidemic cerebro-spinal meningitis, the proportion of well 

 carriers may be much higher, and infant paralysis appears to 

 be chiefly spread in this manner. 



Bacteria are solid particles not easily detached from moist 

 surfaces. Quietly expired air is germ-free, w n . 

 and disease microbes are not transmitted by r 

 the atmosphere except where there is gross Q , 

 local pollution by the spray thrown out in 

 coughing or sneezing or by clouds of infected dust. Cloth- 

 ing, books, toys, or other objects handled by the infected 

 person play a much smaller part in the spread of disease 

 than was thought a few decades ago. There is no absolute 

 sharp line to be drawn between objects of this sort, which are 



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