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POPULAR SCIENCE MONTHLY. 



which are contagious; while malarial fevers, typhoid fever, yellow 

 fever, cholera, pneumonia, peritonitis, etc., are infectious diseases 

 which are not contagious — at least, they are only contagious under 

 very exceptional circumstances, and those in close communication 

 with the sick as nurses, etc., do not contract these diseases as a result 

 of such close association or contact. 



The generalization that all infectious diseases are due to the in- 

 troduction into the bodies of susceptible individuals of living germs 

 capable of reproduction is based upon exact knowledge, gained 

 chiefly during the past twenty years, as regards the specific infectious 

 agents or germs of a considerable number of the diseases of this class. 

 In some infectious diseases, however, no such positive demonstration 

 has yet been made. 



The investigations which have been made justify the statement 

 that each infectious disease is due to a specific — i. e., distinct — 

 micro-organism. There are, however, certain infectious diseases 

 which physicians formerly supposed to be distinct, and to which 

 specific names are given which are now known to be due to one and 

 the same infectious agent or germ. Thus puerperal fever and ery- 

 sipelas are now recognized as being caused by the same germ, the 

 germ which is the usual cause of pneumonia is also the cause of a 

 considerable proportion of the cases of cerebro-spinal meningitis, etc. 



In considering the geographic distribution of infectious diseases 

 we will find it necessary to divide them into two groups, one in 

 which the specific infectious agent or germ multiplies only within the 

 bodies of infected individuals, the other in which it also multiplies 

 external to the bodies of infected individuals when conditions as to 

 temperature, moisture, and organic pabulum are favorable for such 

 external multiplication. 



In the first group we have all those diseases which are trans- 

 mitted only by personal contagion, direct or indirect — i. e., by con- 

 tact with the sick or with articles infected by such contact (fomites). 

 This list includes smallpox, chicken pox, measles, scarlet fever, 

 mumps, whooping-cough, influenza, and diphtheria. 



In the second group we have cholera, typhoid fever, yellow fever, 

 and the malarial fevers. 



It is evident that the geographic distribution of diseases of the 

 first group will depend chiefly upon conditions relating to the sus- 

 ceptibility of different races of mankind, their knowledge of pre- 

 ventive measures, such as disinfection, vaccination, and isolation of 

 the sick, their mode of life and intercourse with each other and with 

 peoples occupying different geographic areas, etc. Nomadic sav- 

 ages, or people living upon islands remote from the channels of 

 commerce, are less liable to suffer from infectious diseases of foreign 



