6 THE AMERICAN MONTHLY [January, 



plants is equally convincing. Several observers noticed that the blood 

 and other tissues of animals suffering from the disease known as anthrax 

 or malignant pustule contained small rod-like bodies, and it was sup- 

 posed that these were in some way the cause of the disease. Inocu- 

 lation of traces of blood or tissues from affected animals was always 

 followed by the disease, but how was it possible to separate out the 

 little rods from the other things contained in the blood ? If this could 

 be accomplished, they could be tried upon animals, and if they produced 

 the disease the proof would be conclusive. This is just what Koch 

 accomplished. He found that the little rod-like bodies grew very well 

 outside of the body, and by cultivating them through many generations 

 he freed them from anything which might have been clinging to them 

 in the blood. The only thing which his cultures contained were the 

 little rods which had descended from those in the blood. Now he found 

 his cultures to be just as virulent as the blood, etc., from an animal 

 suffering from anthrax, which proves conclusively that it is these little 

 rod-shaped bacteria which cause the disease. 



I have cultures obtained from Koch's laboratory, and can produce an- 

 thrax in mice, guinea pigs, rabbits, etc., by inoculating the smallest 

 trace. Not only has this been proved for anthrax, but for many other 

 diseases as well. But if I inoculate with such a small amount, how is • 

 it that bacteria are found in all the organs and tissues? The answer is 

 evidently that the bacteria have multiplied enormously in the animal. 



Fermentation has been proven to be due to a yeast fungus as con- 

 clusively as infectious diseases and decomposition have been shown to 

 be due to bacteria. 



Although we are so positive at the present time that we know the 

 cause of many infectious diseases, of decomposition, and of fermentation, 

 it has not been many years since the whole subject was looked upon 

 with skepticism by men whose opinion was of weight. Still, for at 

 least 230 years the idea that infectious diseases are caused by a living 

 contagion has been entertained by men of learning. But the deductions 

 of the advocates of the theory were more philosophical speculations 

 than facts proven by experiment, and the whole subject fell into disrepute. 

 There was about it so much that appeared vague and intangible, and 

 even ludicrous, to medical men of 150 years ago, that in 1726 a comic 

 poem appeared, placing the germ theory and its advocates in such a 

 ridiculous light that it was well into the present century before any- 

 thing like general interest was again aroused. The German anatomist, 

 Jacob Henle, in 1840 expressed the conviction that contagious diseases 

 must be caused by a living microscopic organism, and the weight of 

 his opinion did much to give a new impetus to investigation in this 

 line of research. The reasons why Henle was led to his conclusions 

 are the following :— 



In infectious diseases there is something which is directly or indi- 

 rectly communicated from a sick animal to a well one and causes dis- 

 ease in the latter. It is veiy probable that the thing which causes the 

 sickness does so in very small quantities, because one sick animal can 

 infect a whole herd. It is also invisible to the naked eye. If it were 

 an invisible gas, it would begin to affect the animal at once, whereas 

 we all know that a certain time always elapses between the exposure 

 and the breaking out of the disease. If you bring a sick animal into a 



