INTRODUCTION 



There are many aspects of life which, when viewed retro- 

 spectively, fall into a pattern, although in their developing 

 they seemed but a random play of accident and circumstance. 

 I think this holds for the professional life of many scientists. 

 In any long career of research we tend to move gradually, or 

 on occasion precipitately, from one topic of interest to another. 

 The moves seem to be made for a variety of reasons, the 

 most cogent, that it is the line along which our current work 

 has developed; but more often than not that development 

 was itself influenced by pressures from work done in a dozen 

 different laboratories or by the advent of a new instrument or 

 technique. In one way or another a scientific career unfolds 

 until almost unexpectedly one finds oneself a senior worker 

 with thirty-five years of experience in the laboratory and one 

 is bidden to seek a central theme on which to illustrate signi- 

 ficant aspects of one's personal research interests. 



Looking back under these circumstances it is nearly 

 always possible to find that central theme. In my own case 

 I think it is perhaps easier than for most scientists. It is almost 

 an automatism that when I have been struggling to find 

 a title for some lecture or article on a general topic the first 

 that came to mind was nearly always ' A biological approach 

 to — '5 or 'The natural history of (infectious disease, cancer, 

 war, woman or what not) '. 



I have always been fascinated by the simple concepts of 

 biology — reproduction, mutation, sexual redistribution of 

 characters and selective survival — and how the interactions 

 of species and ecological communities can be brought into 

 a comprehensible pattern in the light of those concepts. 

 Particularly for the problems of infectious disease in the 



