The germs which cause these diseases are given off and escape from
the sick person in various secretions and discharges. This includes
discharges from the throat, nose, ears and eyes; pus from abscesses
in the neck; sputum or other excretions. The crusts from the skin
lesions of chicken pox contain the germs of the disease, but the scales
from the skin in measles and scarlet fever do not usually do so. The
amount of secretion or discharge required to carry enough germs to
cause infection is very minute. When the germs are once located on
suitable soil they multiply rapidly and enormous numbers are soon
produced from an original few. In originating new cases of disease the
secretion from the sick with its germs is deposited on some part of
the mucous membrane, or lining of the respiratory tract, as the lining
of the nose, throat or larynx; or enters through the mouth, and, being
swallowed, lodges in the stomach or intestine. At times the infecting
material enters through wounds and injuries of the skin, the intact
healthy skin usually forming a perfect protection against infection.
The passage of secretions from the sick person to others is
accomplished in numerous ways. Sometimes this is through direct
contact between two persons, as in kissing. More often the contact is
indirect, the secretion being carried on some object. Anything that
is contaminated by secretions may carry them to a second person. A
few of the most common carriers, such as hands, clothing, bedding,
eating utensils, cups, forks and spoons, toys and pet animals may
be mentioned. During forced expiratory efforts, such as coughing,
sneezing, hawking, stuttering, loud talking or crying, small particles
of secretions from the throat and mouth are thrown into the air in the
form of what is known as mouth spray. This may be inhaled by persons
who are near and be deposited in the throat or nose. This manner of
transferring contagious diseases is not so frequent as the others
mentioned, and only occurs at distances of a few feet. Secretions
which become dry and pulverized into dust outside the body soon lose
their power of infecting. Dust is not of much danger as a means of
transferring contagious diseases. Sometimes various foods which are
contaminated by disease discharges serve to carry them to well persons,
in which case they may be deposited in the throat or pass into the
stomach or intestines. This is specially true of milk, which has been
responsible for many outbreaks of scarlet fever and diphtheria. The
disease germs in the milk do not come from cows, but get into the milk
during or after milking from the hands, sputum and other means of
contact of the persons who handle the milk.