228 HUMAN DISEASES 



immune, or death, unless he possesses a high degree of resisting 

 power for example, the microbes of measles, common cold, 

 whooping-cough, and tuberculosis in England, malaria in West 

 Africa, and dysentery in India. 



378. The species of microbes that cause disease in man, 

 numerous as they are, form but a fractional part of the total 

 number of parasitic species which afflict animals and plants, and 

 these again are few as compared to the multitudes of unicellular 

 organisms which find their nutriment in non-living matter. Some 

 of the latter, the saprophytes, exist on the dead bodies of animals 

 and plants, which but for them would cumber the earth unchanged. 

 Each species fits its own particular niche in nature. All are 

 capable of extremely rapid multiplication, a circumstance which 

 not only secures the persistence of the species, but provides 

 material for selection, and, therefore, facilities for quick adaptation 

 to changing environments. 



379. Of the microbes which cause 'human disease, some, for 

 example those of typhoid, cholera, dysentery, and diarrhoea, enter 

 the body in water or other nutritive substances. Since infected water 

 is a very common vehicle, they are usually termed water-borne. 

 The microbes of other species are commonly inhaled. The types 

 which use this latter method of entrance are usually very minute ; 

 they float in the air like fine dust, and therefore are termed air- 

 borne. Such are the microbes of influenza, smallpox, common 

 cold, and many other maladies. Few of these types, owing 

 doubtless in part to their small size, have been seen. While, 

 seemingly, the microbes of all air-borne species are very minute, 

 small size is of course not their only means of adaptation to their 

 method of infection. For example, it appears likely that their 

 mode of existence in the body is such that they are exhaled 

 directly into the atmosphere by the breath of the sufferer, 1 where- 

 as the microbes of the water-borne maladies pass from him mainly 

 in solid or liquid excreta. The air-borne microbes, owing to their 

 mode of conveyance and their enormously rapid rate of multiplica- 



1 There is evidence that the microbes of such diseases as scarlet fever may be 

 carried into the atmosphere, or may infect the clothes through dried particles 

 from the surface, especially in the later stages of the disease. In the latter case, 

 the microbes are beyond the reach of the phagocytes or the enzymes (see 385-6). 

 This is probably the explanation of the well-known fact that a person recovered 

 from scarlet fever is often infective to his fellows for a period which is long com- 

 pared to that which is the rule with common cold or influenza. In its earlier 

 stages it is practically certain that scarlet fever infects through the breath. 

 Certainly^it is very infective before there is any desquamation of the skin. 



