CHAPTER XIV 

 EPIDEMIC AND ENDEMIC DISEASE 



The conditions of parasitic life Probably every disease originated from a 

 single centre No new diseases are known Disease in ancient and modern times 

 Contagious diseases Insect-borne diseases Air-borne diseases Water-borne 

 diseases Earth-borne diseases The antiquity of diseases Pestilence The 

 invasion of the Western Hemisphere by disease Diseases as empire-builders. 



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440. "^""^f 7"E mav safely assume that the micro-organisms of 

 disease evolved from saprophytic forms of life, the 

 only alternative being the impossible hypothesis 

 that these highly specialized types arose by spontaneous generation 

 after the evolution of the higher animals and plants. Precisely 

 how the passage from a saprophytic to a parasitic mode of life 

 occurred is a matter of conjecture. We know only that there is per- 

 petual war in nature, and that the bodies of the higher animal types 

 contain nutritious material of which the lower types must always 

 have tended to avail themselves. From the beginning, therefore, 

 there must have been some sort of a struggle. The evolution of 

 the parasite had two concurrent phases. First, the saprophytic 

 species must have become capable of persisting for a longer or 

 shorter period in living bodies in spite of their defences. Second, 

 it must have evolved means of passing from infected to healthy 

 but susceptible individuals. Some saprophytic species are very 

 virulent, for example those of putrefaction. Apparently the 

 toxins they manufacture are either digestive ferments or means of 

 offence or defence against other lowly organisms. Under favour- 

 able circumstances they readily adopt a parasitic mode of life, 

 causing disease ; but, since they are unable to pass easily from one 

 individual to another, they cannot maintain themselves as parasites. 

 Thus, also, the microbes of vaccinia are unable to persist unless 

 artificially propagated. The main difficulty to be overcome by an 

 evolving microbic species, then, is not the development of virulence, 

 but the evolution of faculties for passing from one individual to 

 another. Possibly each species of pathogenic organism first estab- 

 lished itself as a regular parasite during times when hardship, famine, 



