en one or more of these factors is unfavorable,
development of germs is checked; if unfavorable conditions are extreme
or long continued, the organisms begin to die. It is difficult to say at
exactly what moment they will die if deprived of moisture or exposed to
extremes of temperature or other unfavorable conditions, just as it
would be impossible to state at exactly what moment a collection of
house plants would all be dead if water were withheld, or if the room
temperature were greatly reduced.

Most pathogenic organisms, however, do not flourish long outside the
body, and owe their continued existence to a fairly direct transfer
from person to person. They gain access to the body through mucous
surfaces such as the respiratory and digestive tracts, and through
breaks in the skin, such as cuts, abrasions, and the bites of certain
insects. They leave the body chiefly in the nasal and mouth discharges,
as in coughing, sneezing, and spitting, in the urine and bowel
discharges, and in pus or "matter."

[Illustration: FIG. 6. (_L. H. Wilder._)]

The problem of controlling communicable diseases, consequently, lies in
preventing the bodily discharges of one person from travelling directly
into the body of another. If a person is not expelling pathogenic germs,
it is clear that he cannot pass diseases on to others. But both
pathogenic and harmless germs follow the same routes from person to
person, so that safety as well as decency lies in preventing so far as
possible all exchanges of bodily discharges.

There are five routes by which the bodily discharges most frequently
travel from one person to another. Four of these routes of infection are
called public, because in most cases efforts of individuals alone are
not sufficient to control them. The public routes are water, milk, food,
and insects. The fifth, or private route, includes all means by which
fresh discharges of one person are passed to another, as when nose and
mouth discharges are carried in coughing, sneezing, and kissing, or when
bowel and bladder discharges are carried by the hands. These five routes
in a given case differ greatly in relative importance, but the fifth, or
direct route plays an immense part, although its importance in causing
sickness has only lately been recognized. It cannot be too strongly
emphasized that the chief agent in the spread of human diseases is man
himself, and the human hand is the great carrier of disease germs both
to and