8 THE POPULAR SCIENCE MONTHLY 



industry of France, he again applied his microscope to the task, demon- 

 strated the presence of living corpuscles in the bodies of the moths, whose 

 offspring later succumbed to the disease, and saved the silk cultivators 

 by a quarantine based on the destruction of eggs from such infected 

 parents. In his later studies of anthrax and chicken cholera, he demon- 

 strated that diseases of the higher animals too were due to specific 

 microbes; and with his work and its extension by Eobert Koch the 

 mystery which for centuries had shrouded the communicable diseases was 

 at last solved. Each case of sickness of this kind is a definite infection 

 with a specific microscopic germ, which grows in the body as a mold 

 grows in a jar of jelly and in its growth produces chemical poisons, 

 which cause the weakness, pain, fever, delirium and the other manifesta- 

 tions of disease. The self-limited nature of such maladies is due, as 

 Pasteur too showed, to the fact that the body cells react against the in- 

 vaders in a specific and purposeful manner, which, if they finally tri- 

 umph, leads to a more or less lasting state of immunity. The spread 

 of communicable disease in the community is no longer a "pestilence 

 that walketh in darkness," but the transfer in tangible ways of a small 

 but definite animal or plant ; and its control can be confidently looked 

 for from the study of the life histories of these microscopic organisms 

 and the working out of practical methods which shall prevent them from 

 gaining access to our bodies. 



The communicable diseases are merely striking examples of the more 

 general biological phenomenon of parasitism described by Swift in the 

 famous and often misquoted lines : 



So naturalists observe, a flea 

 Has smaller fleas that on him prey; 

 And these have smaller still to bite 'em 

 And so proceed ad infinitum. 



It should be remembered that the word parasite was first coined for 

 members of the human species. The parasite in Grecian times was the 

 one who " sat beside " the great man, the hanger-on in the palace of the 

 prince, who gained a precarious living at the expense of his complaisant 

 host. We may hope that the type is less common in modern times and 

 it is fair to remember that in the microbic world, as in our own, the para- 

 site is an exception rather than the rule. The great majority of the 

 bacteria are honest, industrious, useful citizens, who ripen our cream and 

 butter and cheese, make our vinegar and lactic acid, dispose of our waste 

 materials and play a most important part in maintaining the fertility of 

 the soil. The tubercle bacillus and the malaria germ, like the thief and 

 the murderer are perverted individual representatives of a generally 

 sound stock. 



There is another very important point of resemblance between the 

 disease germ and the human parasite. Just as the man who has learned 

 to live at the expense of society soon loses the capacity to do an honest 



