380 



POPULATIONS 



that infectious disease can be thought of 

 with profit along ecological lines as a 

 struggle for existence between man and 

 microorganisms of the same general qual- 

 ity as many other types of competition 

 between species in nature" (Burnet, 1940). 

 "All living things have an ecology, and 

 those producing disease are no exceptions. 

 Some of the viruses may not be alive, yet 

 their ecology, if one is permitted to use the 

 word in this connection, is so similar to 

 that of living things that they may for the 

 purpose of this discussion be considered 

 collectively with other infectious agents. 

 . . . The behavior of infectious diseases in 

 a population is nothing more than an ex- 

 pression of conflicts between various forms 

 of life in an efi^ort to arrive at a satisfactory 

 equilibrium. ... At least, my approach 

 to epidemic diseases at the present time 

 will be along biological and ecological 

 lines" (Rivers, 1947). 



These two quotations serve two func- 

 tions as an introduction to our brief treat- 

 ment of host-parasite interactions. First, 

 they indicate that the modem medical 

 epidemiologist views this problem in the 

 same fundamental way as does the ecol- 

 ogist. The principal difi^erence, apart from 

 technical considerations, between the two 

 groups of workers is that the epidemiolo- 

 gist of necessity works chiefly with one 

 species of host population (man) and 

 limits himself to parasites that are patho- 

 genic upon that host and that, to a con- 

 siderable extent, induce morbidity instead 

 of mortality. The emergent principles are 

 essentially similar in both cases. Owing to 

 the nature of his methods the clinical epi- 

 demiologist must deal largely with blocks 

 of data that accumulate after an epidemic 

 has run its course ("descriptive epidemiol- 

 ogy"). That is to say, he rarely has any- 

 thing to say in advance about the controlled 

 planning of the investigation. His method 

 therefore is almost exclusively statistical.* 



* W. H. Frost ( 1927) published an excellent 

 essay that defines comprehensively and philo- 

 sophically the science of epidemiology while 

 at the same time critically discussing, then de- 

 fending, the "circumstantial" character of 

 epidemiological data. About the latter point 

 Frost says: "Given sufficient scope and accuracy 

 of observations, a conclusion as to the nature 

 and spread of a disease may often be estab- 

 lished quite firmly by circumstantial evidence 

 well in advance of experimental confirmation." 



The population ecologist, as exemplified 

 particularly by certain economic entomolo- 

 gists, utilizes the statistical approach as 

 well, but frequently employs experimenta- 

 tion as an additional analytical tool. Of 

 recent years investigators motivated by 

 chnical considerations have also turned to 

 experimentation with notable reward. An 

 excellent illustration is the work of Green- 

 wood, who established epidemics of various 

 diseases in colonies of laboratory rodents, 

 studied the course of such epidemics under 

 controlled conditions, and, observed host 

 mortality (see especially Greenwood, Hill, 

 Topley, and Wilson, 1936, and Wilson, 

 1945), The second point made by Burnet's 

 and Rivers' quotations is the obvious impli- 

 cation that both the ecologist and the epide- 

 miologist have much to learn from each 

 other. 



In addition to the two general ap- 

 proaches to host-parasite population inter- 

 actions, there is also a theoretical, and 

 largely mathematical, aspect. This is dealt 

 with as a special instance of the interspe- 

 cies competition problem by such authors 

 as Lotka and Volterra (see pp. 271, 326, 

 and 367). It has also received extended 

 and more numerical treatment by Nichol- 

 son (1933) and Nicholson and Bailey 

 (1935) and by Thompson (1939), who 

 present somewhat divergent viewpoints. 



Nicholson and Bailey attempted to for- 

 mulate a comprehensive theory dealing 

 with the competition that develops within 

 animal populations, both within the same 

 species and between distinct species, as 

 they search for various necessities of life 

 along with the relation of such competi- 

 tions to population growth form. Thomp- 

 son examines these ideas and is critical of 

 certain of them, both on methodological 

 and theoretical grounds. An extended dis- 

 cussion of their disagreement cannot be at- 

 tempted here. There is, however, one basic 

 assumption underlying the theory of Nich- 

 olson and Bailey, and attacked by Thomp- 

 son, that, because of its general ecological 

 interest, merits our attention. This is the 

 hypothesis of "random searching." After 

 making the obvious point that all organ- 

 isms must obtain food, mates, and suitable 

 niches in which to live, Nicholson and 

 Bailey conclude that these are found by 

 populations through a process of random, oi 

 completelv unorganized, search. They con- 

 clude further that since organized search 



