138 ANIMAL LIFE AND HUMAN PEOGRESS 



Tliese fantastic theories received their death-blow from 

 the application of zoological methods and ideas about fifty 

 years ago. The microscope revealed for the first time the 

 fact that the parasitic worms produce enormous numbers of 

 e^s. Obviously, if these milUors of eggs could develop and 

 attain their maturity in the intestine in which they were laid, 

 the imhappy host which harboured them must perish with all 

 his parasites. Necessarily, therefore, the eggs must be dis- 

 charged to the outside. Consideration of the further fate of 

 these eggs brought about a revolution in current views on 

 the source of parasitic infections. It was seen that danger 

 did not arise from within but, through contamination of the 

 environment, from without. It was reahsed that the parasites 

 of man were not a peculiar people of himian origin but part 

 of the fauna of the outside world, which had, for purposes of 

 better food and shelter or for the propagation of their species, 

 adapted themselves to a parasitic habitat within the human 

 body during a phase of their life-cycle. 



We know now that the sexually mature worms cannot 

 reproduce a new generation of adults within their host. Each 

 adult worm must have entered the body as a microscopic 

 form. We know also that, with a possible single exception,^ 

 the eggs or young of these parasites cannot immediately infect 

 another person after they have passed out of an infected 

 person. A period of delay must first intervene before the 

 young forms attain sufficient maturity to become " infective." 

 The changes which take place in the young parasite during 

 this period are such that the resulting " infective stage " has 

 no likeness to the newly born. Similarly it bears no resem- 

 blance to the adult into which it will eventually develop. 



As long as the infective stage could not be recognised the 

 physician had no means by which he could prevent the spread 

 of the disease except by destroying the parasites through the 

 administration of drugs to all infected persons ; by securing 

 the control and safe disposition of all human excrement, and 

 by enforcing the sterilisation of all food and drink not collected 

 from sources free from the possibility of contamination. 

 » Oxyuris vRrmicularis, the " seat " worm common in children. 



