PATHOGENICITY 825 



tamoeba histolytica (who did not develop dysentery) and "convalescent" 

 carriers (who have suffered with dysentery, but have become convales- 

 cent). In fact, they passed a strain of the amoeba from a convalescent 

 carrier serially through three other men, two of whom became contact 

 carriers, i.e., did not develop dysentery, and one of whom became a 

 victim of an acute attack of amoebic dysentery. Meleney and Frye, in 

 the experiments previously mentioned, found that kittens inoculated 

 with the same strain differed as to whether or not they became infected, 

 as to the extent and severity of the lesions in the colon, and as to the pe- 

 riod of survival of the diseased kittens. 



Individuals differ also in the degree of resistance offered to the multi- 

 plication of the malaria parasite in their blood and tissues, and in their 

 reaction to parasite density. The existence of racial tolerance or resist- 

 ance of Negroes to inoculation with Plasmodium vivax was pointed out 

 by Boyd and Stratman-Thomas (1933b), though it was by no means 

 absolute. The same authors later (1934) reported their finding that 

 Caucasians appear to be universally susceptible. Wilson (1936, quoted 

 by Hackett) made the observation that Bantu babies in Tanganyika Ter- 

 ritory were all infected with the three species of human malaria by the 

 fifth month of life, and commented as follows : 



One of the striking features of this period of acute infestation, lasting about 

 eighteen months, is the difference in degree of infestation in different in- 

 dividuals. These babies were constantly being reinfected by fresh invasions 

 of sporozoites. The difference cannot therefore be due to variations in the 

 parasites, but rather to a variation in individual resistance. 



Hackett (1937) discusses the variability in the incubation period ex- 

 hibited by different individuals, and states that in some cases there were 

 as few as one parasite per cubic millimeter at the onset of symptoms, while 

 in others there were 900. 



Thus it is evident that the clinical aspects of protozoan infections 

 may differ, owing to inherent basic characters of both the parasite and 

 the host. The reaction of the host may be governed further by another 

 factor that we shall designate the physiological state. Admittedly very 

 little is known concerning the relationship between the physiological 

 state and pathogenicity, but one would conclude a priori that a far-reach- 

 ing relationship should prevail here. As a striking concrete example, 

 nursling rats usually succumb to the long-supposed "non-pathogenic" 



