vi PREFACE 



dealing with infection and immunity and with methods of biological 

 diagnosis and treatment of disease, so that the student and practi- 

 tioner of medicine, by becoming familiar with underlying laws and 

 principles, may not only be in a position to realize the meaning and 

 scope of some of these newer discoveries and methods, but may be 

 in better position to decide for themselves their proper application 

 and limitations. 



We have not hesitated, whenever necessary for a proper under- 

 standing of processes of bacterial nutrition or physiology, or for 

 breadth of view in considering problems of the relation of bacteria 

 to our food supply and environment, to make free use of illustrations 

 from the more special fields of agricultural and sanitary bacteriology, 

 and some special methods of the bacteriology of sanitation are given 

 in the last division of the book, dealing with the bacteria in relation 

 to our food and environment. 



In conclusion it may be said that the scope and arrangement of 

 subjects treated of in this book are the direct outcome of many years 

 of experience in the instruction of students in medical and in advanced 

 university courses in bacteriology, and that it is our hope that this 

 volume may not only meet the needs of such students but may prove 

 of value to the practitioner of medicine for whom it has also been 

 written. 



It is a pleasure to acknowledge the courtesy of those who furnished 

 us with illustrations for use in the text, and our indebtedness to Dr. 

 Gardner Hopkins and Professor Francis Carter Wood for a number 

 of the photomicrographs taken especially for this work. 



P. H. H., Jb., 

 H. Z. 



