PREFACE TO THE THIRD EDITION. 



Since this work first appeared, extensive progress has 

 been made in the subjects of which it treats, and consid- 

 erable valuable literature has been added, making it 

 necessary to materially increase the size of the book. 



The gradual but steady specialization that has taken 

 place in bacteriology, especially in relation to the public 

 health and industries, has given us so much information 

 upon the infectious diseases of man, their etiology, diag- 

 nosis, and treatment ; the infectious diseases of the lower 

 animals and their danger to man ; the proper source and 

 preparation of water for public use; the disposal of 

 sewage ; the protection of the consumer against polluted 

 milk ; the means of artificially ripening cream and 

 flavoring butter ; and the protection of canned goods 

 from contamination during manufacture, that it has 

 become a serious problem to know how much can safely 

 be left out, and just what must be put in, a text-book. 

 That author is indeed to be congratulated who can satisfy 

 his readers in a limited space ! 



In preparing this edition the primaiy object of the book 

 has not been abandoned, but the endeavor has been made 

 to carry out as consistently as possible the original plan 

 of a work upon Pathogenic Bacteria. 



Some bacteria harmless in themselves have been con- 

 sidered because of their confusing resemblances to patho- 

 genic forms. As nearly as possible, space has been ap- 

 portioned according to the importance of the subjects 

 dealt with. 



The matter upon Infection and Immunity has been 

 entirely rewritten, and is presented in what is hoped will 



