vive as long as two weeks. If dried, but kept in the dark, it
will survive four hours. If exposed to sunlight, or even diffuse
daylight, it dies within an hour. In other words, under the conditions
of dampness and darkness which often prevail in crowded tenements it may
remain alive and malignant for weeks; in decently lighted and ventilated
rooms, less than two hours. This explains why, in private practice and
under civilized conditions, epidemics of this admittedly infectious
disease are rare; while in jails, overcrowded barracks, prison ships,
and winter camps of armies in the field they are by no means uncommon.
This is vividly supported by the fact brought out in our later
investigations of the sputum of slum-dwellers, carried out by city
boards of health, that the percentage of individuals harboring the
pneumococcus steadily increases all through the winter months, from ten
per cent in December to forty-five, fifty, and even sixty per cent in
February and March. The old proverb, "When want comes in at the door,
Love flies out at the window," might be revised to read, "When sunlight
comes in at the window the pneumococcus flies 'up the flue.'"

Authorities are still divided as to the meaning and even the precise
frequency of the occurrence of the pneumococcus in the healthy human
mouth. Some hold that its presence is due to recent infection which has
either been unable to gain entrance to the system or is preparing its
attack; others, that it is a survival from some previous mild attack of
the disease, and the body tissues having acquired immunity against it,
it remains in them as a harmless parasite, as is now well known to be
the case with the germs of several of our infectious diseases--for
instance, typhoid--for months and even years afterward. Others hold the
highly suggestive view that it is a normal inhabitant of the healthy
mouth, which can become injurious to the body, or _pathogenic_, only
under certain depressed or disturbed conditions of the latter. In
defense of this last it may be pointed out that dental bacteriologists
have now already isolated and described some thirty different forms of
organisms which inhabit the mouth and teeth; and the pneumococcus may
well be one of these. Further, that a number of our most dangerous
disease germs, like the typhoid bacillus, the bacillus of tuberculosis,
and the bacillus of diphtheria, have almost perfect "doubles,"
law-abiding relatives, so to speak, among the