CHAPTER XIII 



THE ORIGINS OF ZYMOTIC DISEASES 



The microbes of disease are an infinitesimal portion of the total number 

 of bacteria Men acquire microbic diseases as a rule only from 

 other men The conditions necessary for the evolution of a microbic 

 disease The antiquity of microbic diseases Their first origin 

 Their spread Earth-, air-, and water-borne diseases Epidemic and 

 endemic disease Epidemics in the ancient and modern worlds. 



285. WHEN we attempt to solve the problem of the origin 

 and evolution of zymotic diseases we are lost at once in a 

 sea of conjecture. It is possible, however, to draw certain 

 deductions which are not devoid of a high degree of prob- 

 ability. The pathogenetic microbes form an infinitesimal 

 fraction of the total number of bacterial species. In nature 

 it is not easy to find a speck of soil or a drop of water 

 that is entirely sterile. Most species of bacteria are in- 

 nocuous ; that is, they are capable of a saprophytic exist- 

 ence only. It is a safe assumption that all parasitic forms 

 are derived from saprophytic ancestors ; otherwise we should 

 have to uphold the belief, now very generally abandoned, 

 that the highly specialized bacteria of human diseases 

 arose by spontaneous generation after the evolution of 

 the higher animals. It would be vain to speculate on the 

 steps by which this or that species of pathogenetic microbe 

 passed from a saprophytic to a parasitic mode of life. Possibly 

 in some cases the passage was from dead organic material to 

 healthy living beings by way of feeble or dying individuals 

 in whom the power of resistance had been lowered, perhaps 

 by famine. Whatever the steps of the transition, each species 

 that became parasitic evolved special means of offence and 

 defence, and special means of passing from host to host, and 

 so securing the persistence of the species. Presumably all 

 these changes were brought about by the agency of Natural 

 Selection. 



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