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 zoölogy and botany; it has given birth to the sciences of
bacteriology and protozoölogy 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. It was
early recognized that there were diseases which differed in character
from those generally prevalent; large numbers of people were affected
in the same way; the disease beginning with a few cases gradually
increased in intensity until an acme was reached which prevailed for a
time and the disease gradually disappeared. Such diseases were
attributed to changes in the air, to the influence of planets or to