PARASITISM 137 



with vegetable parasites, such as bacteria, yeasts, and allied organisms. 

 It often happens that a parasite in one host may be morphologically 

 indistinguishable from one in another, yet experimentally it is impossible 

 to produce cross-infections. Whether such biological races are to be 

 regarded as distinct species or not is a problem which still requires solu- 

 tion. From the strictly zoological point of view they should be regarded 

 as belonging to one. This highly developed specificity of Protozoan 

 parasites may be kept in mind when organisms of a doubtful nature are 

 being dealt with. The group of parasites known as Toxoplasma, which 

 most observers regard as Protozoa, may actually be vegetable organisms, 

 for it has been found that they are inoculable into a variety of different 

 hosts. 



Another feature exhibited by parasites is one which is termed increase 

 in virulence. Here, again, illustrations occur amongst the trypanosomes. 

 T. gatnbiense can be inoculated from man to laboratory animals. In the 

 first passage the infection may be of slow development, but with successive 

 passages through these animals a strain wall develop which in its behaviour 

 differs from that originally introduced. Whereas at first it may have 

 taken a year to kill the animal in which the trypanosomes were always 

 scanty, finally it brings about a fatal issue in two or three weeks, the 

 trypanosomes reproducing rapidly till the blood of the animal is teeming 

 with them. It is evident that during successive passages the trypano- 

 somes have gradually adapted themselves to these animals. In the case 

 of naturally occurring infections, wdiich are characterized normally by 

 a balance between host and parasite, occasionally infections occur in 

 which such a balance does not exist. In naturally occurring malarial 

 infections amongst native children exposed to the bites of infected 

 mosquitoes there is a balance between the host and parasite, so that 

 the host appears to be little inconvenienced. Sometimes, however, 

 severe and fatal cases occur, either because the natural resistance of the 

 host is low or because the parasites have become peculiarly virulent. 

 These severe infections are of more frequent occurrence amongst human 

 beings who have come from non-malarial countries and are suddenly 

 exposed to infection. It is often claimed that these cases result from 

 a specially virulent strain of parasite, but it seems more probable that 

 the host is at fault, and that the fluids of the body differ from those of 

 the natural hosts. Another illustration is seen in the case of Entamoeba 

 histolytica. In the majority of cases of infection with this amcBba, the 

 organism produces a minimum of inconvenience to its host, which is 

 known as a carrier, but in a small percentage of cases the balance is 

 broken down and acute symptoms of amoebic dysentery reveal themselves. 

 It is found that the reaction of a host varies with the strain or race of 



