As stated on the title paggthis little book is made up Of the Herter Lectures given in 1916 at the University and Bellevue Medical School, New York. These lectures were published in the Journal of Laboratory and Clinical Medi cine. There has been some demand for re prints, Of Which none were made. I have, there fore, gladly acceded to the request of the pub lishers, The C. V. Mosby Company, Who have Offered to present them in the present form. The researches upon Which they are founded have Occupied much Of my time for many years, and have given me a concept of the nature of infection, quite different from that usually taught. Hoping that, in this form, my studies may be helpful to my fellow-workers in the profession.