Greeks were scientific or not, they aimed to be scientific, and they firmly implanted along with all their error, a zeal for the scientific truth about nature and a confident belief in the possibility of its ultimate attainment.
From Hippocrates down to our own times, then, we find that the explanation of diseases of all kinds has been sought by scientific men, not in the activities of spiritual beings, but in the workings of natural law. Results were almost nil, however, so far as the communicable diseases are concerned, until the nineteenth century when a sudden and rich fruition took place here, as in all fields of the biological sciences, as a result of a simple mechanical discovery, the lens-maker's trick of the achromatic objective, which made possible the modern high power microscope and revealed all at once a new and stupendous world—"The world of the infinitely little."
It is true that Leeuwenhoek and other early naturalists had seen the microbes with their primitive simple lenses. It is true that still earlier, in the sixteenth century, the Veronese physician, Fracastorius conceived the communicable diseases as due to "seminaria contagionum," minute particles capable of reproduction in appropriate media and having many of the attributes we know to be characteristic of the bacteria to-day. The Roman author Varro, in writing on the choice of sites for a farm house cautions the builder against the neighborhood of swampy ground "because certain minute animals, invisible to the eye, breed there, and borne by the air, reach the inside of the body by way of the mouth and nose, and cause diseases which are difficult to get rid of." Nevertheless, so far as any scientific demonstration of their nature was concerned, the communicable diseases remained as much a mystery in 1800 as in 400 B.C.
It was the achromatic objective, perfected about 1840, which first revealed the ubiquity of microbic life and its special richness in connection with the processes of fermentation and decay; and it was of profound moment in the history of medicine and sanitation when Pasteur proved, against the opposition and the ridicule of the great Liebig and a host of lesser critics, that fermentation was the result of the action of microbes, little living things which entered into the fermentable fluids and grew and multiplied there, the fermentation being the result of their powerful chemical secretions. The "little leaven that leaveneth the whole lump" was shown to be a self-propagating plant, and the many desirable and obnoxious decompositions to which sugary fluids are subject, each the result of a special microbe.
The analogy between fermentation and disease must have sprung into many minds, with the hope that the solution of the latter problem too might be found by the study of microscopic life. It was again Pasteur who by his rigid experimental method extracted the truth from the mass of good and bad guesses of those who had preceded him. In his study of the disease which threatened to wipe out the great silkworm
