6 INTRODUCTION 



it, occurs over the greater part of the United States, yet not 

 only are the people unfamiliar with the disease and its cause, 

 but most physicians are unacquainted with it and do not know 

 how to diagnose or treat it. 



The history of modern medicine, so far as infectious diseases 

 are concerned, is nothing more nor less than the history of para- 

 sitology in its broad sense, including bacterial and fungous para- 

 sites as well as animal parasites. Previous to the beginnings of 

 our knowledge of the existence of microscopic parasites, and of 

 the effects produced by them, nearly all diseases were interpreted 

 as visitations from angry deities, as the work of demons or as 

 the effect of supernatural causes. Such ideas are still prevalent 

 in those parts of the world where bacteriology and parasitology 

 have not yet penetrated. 



With the exception of the superficial acquaintance which the 

 ancients had with external parasites and a few parasitic worms, 

 parasitology began about the middle of the 16th century when 

 Fracastorio, an Italian, published his belief that disease was due 

 to invisible organisms multiplying within the body. With the 

 invention of the microscope by the Dutch lens-grinder, Leeu- 

 wenhoek, actual observation of microscopic organisms became 

 possible, and this famous pioneer in science observed, in 1675, 

 " animalculae " in rain-water, putrid infusions, saliva, and 

 diarrheal excretions, and made illustrations of them. Based 

 on these scanty observations, the idea that all diseases were 

 caused by these " animalculse " became rampant during the 

 succeeding century. In 1762 Plenciz, a physician of Vienna, 

 apparently with the tongue of a prophet, expressed the idea 

 that all infectious diseases were caused by living organisms, 

 that there was a special " germ " for each disease, that the 

 incubation period of diseases was due to the time necessary for 

 the infecting organisms to multiply, and that the organisms 

 might be conveyed through the air as well as by direct or indirect 

 contact. In the 18th and 19th centuries there was much con- 

 troversy as to the origin of " germs " and the possibility of their 

 spontaneous generation from putrefying matter. A belief in the 

 origin of living organisms only from pre-existing organisms was 

 first expressed by the Italian Redi, in 1668, but scientific proof 

 of it came much later. Experiments by Spallanzani in 1769, 

 Schulze in 1836, Schwann in 1839, Schroeder and von Dusch in 



