cause for disease since the condition recognized as disease is the
result, usually, not of one but of a series of causes or circumstances
more or less connected and linked together, and in many cases not
obviously associated with the resulting disease. Thus, in records of
death, it is very common to see reported pneumonia as the cause
underlying and fundamental, when the cause was really typhoid fever, the
patient yielding to the former disease because of the enfeebled
condition due to the latter. Again, many children contract diseases like
measles or whooping cough because of reduced vitality due to
insufficient nourishment, lack of clothing, and neglect, and their
illness is said to be due to measles or whooping cough when under
proper conditions of care and attention they would not have the disease
at all. The causes of disease therefore may be divided into two classes,
direct and indirect. In the latter class are to be included such causes
as environment, heredity, age, and occupation. In the former class are
to be found such causes as the introduction of disease germs into the
system; the action of poisons, whether introduced into the alimentary
canal or into the lungs, and such external conditions as excessive heat
and cold and accident.

_Effects of dirt._

At one time it was thought that diseases could spring up in the midst of
dirt, and one of the strong arguments for keeping houses clean, for
removing manure piles, and cleaning up back yards, was the fear that
without such care diseases might be induced in those living near by.
This is possible in a certain sense, but unless the seed or germ of the
disease is present in a pile of dirt there need be no fear of the
disease being developed. There is, however, a probability that by the
organic decay and the consequent pollution of the atmosphere the
vitality, energy, and resistance of the individual in the vicinity may
be weakened.

It is well known, for instance, that prisoners confined in damp dark
cells lose vitality, and when released, have but little of their former
physical strength. In the chapter on Ventilation, it has been shown that
persons confined in a small room and breathing their own exhaled air may
in time become unconscious and die, and therefore it is reasonable to
believe that persons living in the immediate vicinity of decaying animal
or vegetable matter will suffer a loss of vitality and will have less
resistance to disease.

_Blood resis