Bacteria are the smallest of microscopic organisms.  They are widely diffused in the natural world, existing independently and also in a parasitical way, in connection with larger forms of organic life.  They multiply with the greatest rapidity.  On the whole, the bacterium fulfills its vital offices in two ways, or with two results; first, fermentation, and secondly, disease.

To this field of inquiry Pasteur devoted himself with the greatest assiduity.  He began to investigate the diseased tissue of animals, and was rewarded with the discovery of the germs from which the disease had come.  It was found that the bacteria of one disease are different from those of another disease, or in a word that the microscopic organisms which produce morbid conditions in animals are differentiated into genera and species and varieties, in the same manner as are the animals, birds and fishes, of the world.  A new realm of life invisible save by the aid of the microscope, began to be explored, and practical results began to follow.

Pasteur at length announced his ability to produce infectious diseases by inoculation; and of this his proofs and demonstrations, were complete.  In the next place he announced his ability to counteract the ravages, of certain classes of diseases (those called zymotic) by inoculating the animal suffering therefrom with what he called an “attenuated” or “domesticated” virus of the given disease.

The matter first came to a practical issue by the inoculation of well animals with the attenuated virus.  The animals so treated became immune; that is, exempt from the infection of the given disease.  Pasteur gave public demonstrations in the fields near Paris, using the disease called splenic fever, and sheep as the subjects of his experimentation.  The whole civilized world was astonished with the results.  The tests were conducted in such a way as to preclude the possibility of error.  It was shown, in a word, that by the simple process of inoculating well animals with the modified poison the infectious disease might be avoided.

It were long to tell the story of the experimentation and discovery that now followed.  The last quarter of the century has been fruitful in the greatest results.  The bacilli of one disease after another have been discovered, and the means have been invented of defending the larger animal life from the ravages of microscopic organisms.

But what is an “attenuated” virus?  Pasteur and other scientists have shown that by the inoculation of suitable material, such as a piece of flesh, with the poison of a given disease, the bacteria on which that disease depends rapidly multiply and diffuse themselves through the substance.  If poison be taken from the first body of infected material and carried to another, that other becomes infected; and from that the third; from the third the fourth, and so on to the tenth generation.