THE BIOLOGY OF HEALTH 149 



made mastery at first more difficult, and eventually 

 easier, is that many of these pests require to spend 

 part of their life in one host and part of their Hfe in 

 another. Thus, the malaria microbe spends part of its 

 life in the mosquito, the sleeping-sickness microbe 

 spends part of its life in the tsetse fly. Apart from 

 cases where a human parasite must pass part of its 

 life within another host, there are many cases where 

 insects or the like act as ' carriers ' or distributers. 

 Thus, the house-fly is a great distributer of typhoid. 



When microbes enter the body, what do they do ? 

 They make and liberate poisons which are often very 

 prejudicial to the tissues. By their activity they may 

 also break down membranes and cause structural in- 

 juries or lesions. They may multiply so enormously 

 that they block up passages, and there are other ways 

 in which they work mischief. Luckily we have two 

 great counteractives — we have a bodyguard of wan- 

 dering amoeboid cells or Phagocytes — really a particu- 

 lar kind of white blood corpuscle — which engulf and 

 digest microbes, and there are several destructive sub- 

 stances or antitoxins in the fluid of the blood which 

 counteract the poisons of the intruders. In various 

 artificial ways it is possible to increase the protective 

 efficiency of both these natural defences of the body. 



In the third place, besides constitutional and mi- 

 crobic diseases there are modificational diseases which 

 are due to peculiarities in nurture. Lead poisoning 

 and rickets in children are two well-known examples. 

 All who work in the gold-mines at Johannesburg get 



