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Nature of Infection.
When the infection is not in the lungs (" non-pulmonary
e.g., in glands, bones and joints) there is no danger to other people
unless there is a discharging sore, and even then the danger is
limited. But when there is active disease in the lungs ("pulmonary"
or " respiratory ") living germs get into the surrounding air when
the affected person breathes and speaks, and the organisms are
sprayed into the atmosphere to a distance of several feet when he
coughs or sneezes. Infection is generally caused by inhaling
infected air, or by swallowing infected food or drink ; but by far
the commonest route of infection is from the air.
Mode of Spread.
Most germs that cause disease cannot live without warmth and
moisture, and they die soon after they leave the human body ;
but the tubercle bacillus is so well protected in its surrounding
fatty envelope that it resists drying, and can remain alive in the dust
or droplets of the air for a considerable time.
Resistance of Infection.
We cannot hope to escape infection, to which we are exposed
daily ; but fortunately most of us are protected by a natural
resistance, and most children by the time they leave school have
acquired a considerable resistance by exposure to repeated small
doses of infection from day to day. But there are many young
children who are subjected to repeated doses of infection long before
they have had time to acquire immunity, while others have their
relative immunity swamped by exposure to massive closes of
infection.
Influence of Heredity.
There is no justification for the old and widely-held belief that
tuberculosis (" consumption") is hereditary. Children born of
tuberculous parents are free from the disease, and if they are
removed from their parents at birth they do not develop it; but
there is abundant evidence that children living with infectious
parents are more liable than other children to contract the disease,
and that the liability increases when the doses of infection are
large and repeated.