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."
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 from the body. If unclean hands could be kept away from the orifices of the body,
particularly the mouth, many diseases would soon cease to exist.
Defenses of the Body
In view of all the dangers from disease-producing germs it may seem surprising that the human race has not long ago succumbed to its invisible enemies. But the body has various defenses by means of which it may prevent invasion, or successfully combat its enemies in case they do gain access.
The unbroken skin is usually impassable to bacteria. Virulent organisms are often found upon the skin of perfectly healthy persons, where they appear to be harmless unless an abrasion occurs which affords entrance into the deeper tissues. Most bacteria breathed in with the air cling to the moist surfaces of the air-passages and never reach the lungs.