MAN'S FIGHT FOR SURVIVAL 



617 



Another parasitic infection caused by the roundworm Ascaris 

 lumhricoides involves a rather large proportion of tlie population, 

 especially in the southern parts of this country where the weather is 

 warmer and presumably the conditions necessary for its develop- 

 ment are more nearly ideal. Prior to 1921 various surveys indicated 

 that the eleven states extending east of the Mississippi River and 

 south of the Ohio, with Texas, had an average infection of 13.8 per 

 cent of the population. Further studies were carried on in 1934 with 

 the result that certain regions in mountainous sections where soil 

 conditions were just right for its spread showed an infection rate as 

 high as 30 per cent. However, it was demonstrated that through 

 the use of suitable sanitary methods this worm can be controlled, 

 as seen by reference to its life history, page 225. 



Most of the parasites mentioned which infect man are intestinal 

 forms. In the midwest, however, bathers at a few summer resorts 

 have encountered a different variety. Certain fork-tailed free- 

 swimming larvae, cercariae, of some of the blood flukes which nor- 

 mally penetrate the skin of some of the lower mammals to invade 

 their blood streams apparently mistake man for their normal host. 

 Fortunately they do not continue 

 their development in this unusual 

 host, although causing an intense 

 itching during and after penetra- 

 tion of the skin, chiefly among 

 susceptible people. 



Parasites Acquired Indirectly by 

 Man 



In the higher as well as in many 

 of the lower organisms which 

 parasitize man, the life history of 

 the invader is often found in two 

 or more different hosts. Contact 

 with the parasite, obviously, is 

 necessary in order to have the 

 disease germs enter the body. 



In the case of protozoan para- 

 sites which affect man and in some cases of bacterial infection a carrier 

 usually becomes necessary in order that the infective organism may 

 reach the interior of the body. Among these carriers there are two 



CLAW FROM TIP OF FOOT 



0»BACILU 



■:^mmmi,^^ 



Diagram to show how bacteria might be 

 carried on the foot of the hou.se fly. 



