viii PREFACE TO THE THIRD EDITION 



has not been conducive to careful leisurely recapitulation, and our plea must be 

 the paradoxical excuse that we have not had time to be more concise. 



To those who have assisted us in various ways, we would express our thanks. 

 We are particularly grateful to Dr. N. W. Pirie for his help with some chapters in 

 Part I, to Professor A. Bradford Hill for his advice on Chapter 43, and to Professor 

 S. P. Bedson, Lt.-Col. R. F. Bridges, Dr. A. Q. Wells, Dr. Joan Taylor, Dr. A. W. 

 Stableforth, Dr. R. Lovell, Miss Nancy Hayward, and many of our former helpers 

 for information on particular problems. To Dr. Stuart Mudd and his American 

 colleagues, to Professor A. D. Gardner, Dr. C. F. Robinow, Dr. S. T. Cowan, Dr. 

 N. G. Heatley, Dr. A. Pijper and the publishers of " Endeavour " we are indebted 

 for a number of electron micrographs and photographs ; to Professor J. R. Marrack 

 for Fig. 32 ; to H.M. Stationery Office for permission to reproduce Figures 32, 34, 

 77, 79, 80 and 81 ; and to Miss Margaret Rees for the preparation of some new 

 diagrams. We should also like to pay our tribute to the library staffs of the London 

 School of Hygiene and Tropical Medicine, the Bureau of Hygiene, University 

 College Hospital Medical School, and the Radcliffe Science Library, Oxford, for 

 their unfailing courtesy and help in the tracing of numerous references . 



G. S. W. 

 June, 1945. 



