THE GERM-THEORY OF BIS EASE. 581 



bacteria are so small and so transparent that they can not be easily, if 

 at all, seen in the midst of animal tissues. This difficulty may be 

 lessened by the use of staining agents, which color the bacteria differ- 

 ently from the animal cells. But it often requires long and tedious 

 trials to find the right dye. The obstacles in the way of the second 

 part of the proposition mentioned are no less appalling. Having 

 found a suspected parasite in the blood or flesh of a patient, we can 

 not accuse the parasite with certainty of being the cause of the dis- 

 ease, unless we can separate it entirely from the fluids and cells of the 

 diseased body without depriving it of its virulence. In some cases it 

 is not easy, if possible, to cultivate the parasite outside of the body ; 

 in other instances it can be readily accomplished. Of course, all such 

 attempts require scrupulous care to prevent contamination from other 

 germs that might accidentally be introduced into the same soil. If we 

 can now reproduce the original disease in other animals by infection 

 with these isolated bacteria, the chain of evidence is complete beyond 

 cavil and doubt. But this last step may not be the least difficult, as 

 many diseases of mankind can not be transferred to animals, or only 

 to some few species. 



If we apply these rigid requirements, there are not many diseases 

 of man whose bacterial origin is beyond doubt. As the most unequivo- 

 cal instance, we can mention splenic fever, or anthrax, a disease of 

 domestic animals, which sometimes attacks man, and is then known 

 as malignant pustule. The existence of a parasite in this affection in 

 the form of minute rods and its power of reproducing the disease are 

 among the best-established facts in medicine. It is also known that 

 these rods form seeds, or spores, as they are termed, in their interior, 

 after the death of the patient, which germinate again in proper soil. 

 These spores are the most durable and resisting objects known in ani- 

 mated nature. If kept in the state of spores they possess an absolute 

 immortality ; no temperature short of prolonged boiling can destroy 

 them, while they can resist the action of most poisons, even corrosive 

 acids, to a scarcely credible extent. 



Another disease, of vastly greater importance to man, has lately 

 been added to the list of scourges of unquestionable bacterial origin. 

 I refer to tuberculosis, or consumption. It is true, this claim is based 

 upon the work of but one investigator Robert Koch. But who- 

 ever reads his original description must admit that no dart of criticism 

 can assail his impenetrable position. Here also a rod-shaped bacillus, 

 extremely minute and delicate, has been found the inevitable com- 

 panion of the disease. With marvelous patience Koch has succeeded 

 in getting the parasite to grow in pure blood, and freeing it from all 

 associated matter. It must have been a rare emotion that filled the 

 soul of that indefatigable man, when he beheld for the first time, in 

 its isolated state, the fell destroyer of over one eighth of all mankind ! 

 None of the animals experimented upon could withstand the concen- 



