industry of France, he again applied his microscope to the task, demonstrated 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 demonstrated that diseases of the higher animals too were due to specific microbes; and with his work and its extension by Robert 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 manifestations 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 invaders in a specific and purposeful manner, which, if they finally triumph, 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 parasite 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
