such as typhoid, cholera,
tuberculosis and many others, are caused by the presence of bacteria in
the body, and it is just as definitely known that such maladies as
malaria and sleeping sickness are caused by animal parasites.

Then there is a long list of other epidemic diseases, such as smallpox,
measles and scarlet fever, the exact cause of which has not been
determined. Many of these are believed to be due to micro-organisms of
some kind, and if so they will almost certainly sooner or later be
found. Curiously enough most of the diseases in this last class and many
of those in the first are contagious, while all that are caused by
animal parasites are, as far as is known, infectious but not contagious.

INFECTIOUS AND CONTAGIOUS DISEASES

It is important that we keep in mind this distinction. By contagious
diseases are meant those that are transmitted by contact with the
diseased person either directly, by touch, or indirectly by the use of
the same articles, by the breath or effluvial emanations from the body
or other sources. Small-pox, measles, influenza, etc., are examples of
this group. By infectious diseases are meant those which are
disseminated indirectly, that is, in a roundabout way by means of water
or food or other substances taken into or introduced into the body in
some way. Typhoid, malaria, and yellow fever, cholera and others are
examples of this class. Thus it is evident that all of the contagious
diseases may be infectious, but many of the infectious diseases are not
as a rule contagious, although some of them may become so under
favorable conditions.

Just one example will show the importance of knowing whether a disease
is contagious or infectious. Until a few years ago it was believed that
yellow fever was highly contagious and every precaution was taken to
keep the disease from spreading by keeping the infected region in strict
quarantine. This often meant much hardship and suffering and always a
great financial loss. We now know that it is infectious only and not
contagious, and that all this quarantine was unnecessary. The whole
fight in controlling an outbreak of yellow fever or in preventing such
an outbreak is now directed against the mosquito, the sole agent by
which the disease can be transmitted from one person to another.

EFFECT OF THE PARASITE ON THE HOST

We have seen how a few parasites in or on an animal do not as a rule
produce any appreciable ill effects. This is of