CHAPTER LXXIV 

 INSECTS AS INTERMEDIATE HOSTS 



440. Malaria. Of all the diseases from which man suffers, 

 malaria is said to be the most widespread, occurring all around 

 the earth as far north and as far south of the equator as mos- 

 quitoes may be found. ^ The disease is caused by any one of 

 three or four species of protozoa related to the ameba and 

 known as the plasniodhnn of malaria. The animal feeds upon 

 the red corpuscles of the blood of its host, and then spondatcs 

 (see p. 294). The spores enter new corpuscles, and the process 

 is repeated indefinitely, greatly weakening the victim and some- 

 times ending fatally. 



The parasites were seen in the blood of patients by the French 

 scientist Alphonse Laveran, working in Algeria. He succeeded in in- 

 fecting subjects with the blood of sick people, but he could not find 

 out how the infection takes place naturally. It took twenty years 

 more of careful research and experimentation to establish the fact that 

 the mosquitoes of the genus Anopheles are the agents of infection. 



Two English physicians, Sir Patrick Manson and Dr. Ronald Ross, 

 helped in the establishment of this important fact by tracing the 

 behavior of the parasite in the bodies of mosquitoes. Finally, in 

 1900, an elaborate experiment was conducted by scientists cooperat- 

 ing in England and Italy. In this experiment a number of volunteers 

 lived in the Roman Campagna, a region that had long been notorious 

 for being full of malaria. But the volunteers lived in houses that were 

 carefully screened against the entrance of mosquitoes. They were 

 also careful not to go out in the evening (when the Anopheles is about) 



^ It has been estimated that in the United States the money cost of malaria 

 has been as much as one hundred million dollars a year. This takes the form 

 of time lost from work, the cost of drugs, nursing, and medical service, the 

 unavailability of much fertile land, and so on. In India this disease kills over a 

 million human beings a year, besides causing untold misery to millions of others. 



404 



