NATURAL IMMUNITY 19 



though experimentally other ticks may become infective. There 

 is obviously a constant possibility of the establishment of the 

 disease in other species of ticks and thus of greatly widening the 

 area affected. 



Temperature is an important limiting factor for those parasites 

 which can be directly influenced by it, as external parasites and 

 those which are free-living during a part of their existence. 

 In Mexico, for instance, human lice are entirely absent from the 

 hot coastal plains, though abundant on the high central plateau. 

 Hookworms, which are free-living in their young stages, are con- 

 fined to a broad strip around the tropical and warm temperate 

 portions of the world, and occur outside these limits only in 

 short-lived epidemics during the warm part of the year. Such 

 parasites as bots and screw-worms are equally exposed to the 

 influence of climate, since they are free-living in the adult stage. 



Some parasites are limited by other environmental conditions. 

 In the case of such intermittent external parasites as mosquitoes, 

 biting flies and cone-noses it is obvious that not only tempera- 

 ture and humidity, but also the presence of suitable breeding 

 places and of suitable haunts during resting times must be neces- 

 sary for their continued existence. Again, the local distribution 

 of hookworms is determined, to a large degree, at least, by the 

 nature of the soil. These worms abound where sandy soil occurs, 

 but are rare or absent where there is only limey or clayey soil. 



Natural Immunity. — As has already been pointed out, when 

 a parasite is introduced into a region where it was previously 

 unknown, or, what amounts to the same thing, new hosts are 

 introduced into its territory, its ravages are usually worse than 

 in places where it has been endemic for a long time. The hosts 

 and parasites of a given region come to a point of equilibrium. 

 The host becomes largely immune to the effects of the parasite, 

 and yet harbors it in sufficient numbers to form a reservoir for 

 it, and thereby acts as a "carrier." In some cases a total or 

 partial immunity is built up in youth, when the power of resist- 

 ance to parasitic invasion is usually high; in other cases it is the 

 result of a long struggle extending through many generations. 

 A good example of immunity acquired in youth is found in the 

 case of yellow fever, and of partial immunity, gained through 

 many generations of adaptation, in the case of hookworms in 

 negroes. The terrible destruction wrought by sleeping sickness 



