322 BIOLOGY AND HUMAN LIFE 



244. Intermediate hosts. Of all the diseases from which man 

 has suffered malaria is said to be the most widespread, occurring 

 all around the earth as far north and as far south of the equator 

 as mosquitoes can be found.^ The disease is caused by any one 

 of three or four species of protozoa related to the ameba and 

 known as the Plasmodium of malaria. The animal feeds upon 

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

 that is, breaks up into a large number of tiny bits of protoplasm 

 called spores. Each spore is capable of resuming life activities 

 under suitable conditions. The spores enter new corpuscles, 

 and the process is repeated indefinitely, greatly weakening the 

 victim and sometimes killing him (see a to e, Fig. 152). 



The French scientist Alphonse Laveran, working in Algeria, succeeded 

 in infecting volunteers with the blood of sick people, but he could not 

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

 careful research and experimentation to establish the fact that mosqui- 

 toes of the genus Anopheles are the agents of infection. In 1900 an 

 elaborate experiment was conducted by scientists cooperating in England 

 and Italy. In this experiment a number of volunteers lived in the badly 

 malarious Roman Campagna in houses that were carefully screened 

 against the entrance of mosquitoes. They were also careful not to go out 

 in the evening (when Anopheles is about) without wearing veils and 

 gloves. Thus they lived through the most dangerous part of the year, 

 from early in July until late in October, a7id not one became sick, al- 

 though many of their neighbors became ififected with malaria during the 

 summer. At the same time some mosquitoes were caught and allowed to 

 suck blood from persons suffering from malaria. These mosquitoes were 

 placed in httle cages and shipped to England. Here two young men who 

 had never suffered from the disease, and who lived in a region where 

 there had been no cases of malaria, allowed themselves to be stung by 

 the suspected mosquitoes. In the course of a few days both developed 

 the characteristic symptoms of the disease. This experiment showed 

 that the night air and the vapors from the swamps of the Campagna 

 were harmless, and that the sting of a mosquito that had once bitten a 



1 The money cost of malaria in the United States has been estimated at one 

 hundred million dollars a year. This cost takes the form of time lost from 

 work, the cost of drugs, nursing, and medical service, the idleness 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. 



