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. 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
the action of offended gods. The priests and charlatans who sought to
excuse their inability to treat epidemics successfully were quick to
affirm supernatural causes. Hippocrates (400 B.C.), with whom medicine
may be said to begin, thought such diseases, even then called
epidemics, were caused by the air; he says, "When many individuals are
attacked by a disease at the same time, the cause must be sought in
some agent which is common to all, something which everyone uses, and
that is the air which must contain at this time something injurious."
Aristotle recognized that disease was often conveyed by contact, and
Varro (116-27 B.C.) advanced the idea that disease might be caused by
minute organisms. He says, "Certain minute organisms develop which the
eye cannot see, and which being disseminated in the air enter into the
body by means of the mouth and nostrils and give rise to serious
ailments." In spite of this hypothesis, which has proved to be
correct, the belief became general that epidemics were due to
putre