THEIR RELATION TO MAN 203 



that undergo no change and which, when implanted in 

 a suitable medium, will continue their growth and 

 increase with unabated virulence. We have yet quite a 

 different class of diseases, also due to microscopic or- 

 ganisms, but of an altogether different type: minute, 

 single celled animals in fact, that live in special body- or 

 blood-cells, but are not capable of completing their en- 

 tire life cycle in a single host. 



The best known example of this sort of infestation is 

 that due to the Plasmodium parasite which produces 

 what is loosely known as malaria. It is not so long ago 

 that almost any sort of indefinite 

 illness was likely to be classed as a 

 "touch of malaria:" now-a-days 

 when a doctor diagnoses "mal- 

 aria" he refers to an affection 



Fig. 94. — Part of one 

 caused by one of two or three of the pseudo-trachea used 



specific organisms that have very -« ^^^--^P'^g °^g^"- 

 definite life cycles and produce 



very definite results. All of them agree in being Spor- 

 ozoa, i.e., animals that reproduce by means of spores, and 

 in that they do not complete their entire life cycle with- 

 in the body of their human host. The parasite producing 

 the ordinary type of tertian malaria or " chills and fever " 

 lives in the red blood-corpuscles of the human body and 

 comes to maturity in such a blood-cell in forty-eight 

 hours. It then breaks up into a mass of minute spores 

 which rupture the cell and are liberated into the blood- 

 scrtmi. In this they float about for a short time, and 

 then each spore makes its way into a sound red blood- 

 corpuscle, and in forty-eight hours is itself mature and 

 in turn reproduces in the same way. As all the parasites 

 come to maturity and liberate their spores at about the 

 same time, this causes a disturbance of the body tem- 

 perature resulting in a chill, followed by a fever when the 



