The plague contributed as much as did the arms of the Turks to the downfall of Constantinople and the Eastern Empire in 1453.

It was the plague which in 1348 overthrew Siena from her proud position as one of the first of the Italian cities and the rival of Florence, and broke the city forever, leaving it as a phantom of its former glory and prosperity.

The work on the great cathedral which had progressed for ten years was suspended, and when it was resumed it was upon a scale adjusted to the diminished wealth of the city, and the plan restricted to the present dimensions.

As a little relief to the darkness the same plague saw the birth of the novel in the tales of Boccaccio, which were related to a delighted audience of the women who had fled from the plague in Florence to a rural retreat.
The knowledge which has come from the study of infectious disease has served also to broaden our conception of disease and has created preventive medicine; it has linked more closely to medicine such sciences as zooelogy and botany; it has given birth to the sciences of bacteriology and protozooelogy and in a way has brought all sciences more closely together.

Above all it has made medicine scientific, and never has knowledge obtained been more quickening and stimulating to its pursuit.
Although the dimensions of this book forbid much reference to the historical development of a subject, some mention must still be made of the development of knowledge of the infectious diseases.