SOME INHABITANTS OF MAN 139 



Such procedure would involve vast expenditure, ceaseless 

 educational propaganda and constant co-operation of all 

 members of the community. Yet, without a knowledge of 

 the conditions governing the life of these parasites outside the 

 body, it would prove of no avail against many of the infections 

 met with in tropical countries. 



Against certain of the parasitic worms drugs are impotent. 

 The disposal of human excrement would not necessarily break 

 the " cycle " of many infections, for zoologists have found 

 that the domesticated animals often harbour the same species 

 of parasites as those found in man, and thus act as " reservoir " 

 hosts, and, again, while some species are undoubtedly taken 

 into the human body in food and drink, some of the most 

 important forms have recently been shown to enter through 

 the skin by their own activity. Moreover, although the 

 progeny of many of the parasitic worms leave the human 

 body in the stools, there are some which pass out in the sputum 

 and the urine, while others are sucked from the blood by biting 

 insects. 



One can best meet danger if one knows whence it must 

 come. By divers zoological methods the naturalist has 

 enabled the sanitarian to recognise with certainty the/^. in- 

 fective stage " of each of the species which cause disease in 

 man. He has been able to show that each parasite has its 

 own special method of approach and that this practically 

 never varies. It has, in consequence, been possible to devise 

 a simple and efficacious control over the ingress of each species. 



In another and still more far-reaching way the naturalist 

 has come to the aid of the sanitarian. We have seen that the 

 physician can only rid his patient of certain of his parasites, to 

 wit, some of those which live in the alimentary canal. Those 

 which inhabit the liver, lungs, glands and blood-vessels are 

 inaccessible to the action of drugs. Moreover, the administra- 

 tion of " anthelminthics " is not without danger, as these are 

 poisons and can therefore only be given in minute doses. 

 During the " period of delay," in which the parasites undergo 

 development outside the body, they are much more suscep- 

 tible to attack. The naturalist has studied in detail the various 



