y actually repel them
and do not suffer them to exist in or on their bodies, or they are
tolerant toward them. In the latter case the parasites live at the
expense of the host, but the host has become used to their being there,
adapted to them, and the injury that they do, if any, is negligible.

But when a new animal comes into the region from some other locality the
parasites may be extremely dangerous to it. There are many striking
examples of this. Most of the people living in what is known as the
yellow fever belt are immune to the fever. They will not develop it even
under conditions that would surely mean infection for a person from
outside this zone. Certain of our common diseases which we regard as of
little consequence become very serious matters when introduced among a
people that has never known them before. The cattle of the southern
states are immune to the Texas fever, but let northern cattle be sent
south or let the ticks which transmit the disease be taken north where
they can get on cattle there, and the results are disastrous.

Another striking example and one that is attracting world-wide attention
just now is the trypanosome that is causing such devastation among the
inhabitants of central Africa. With the advent of white men into this
region and the consequent migration of the natives along the trade
routes this parasite, which is the cause of sleeping sickness, is being
introduced into new regions and thousands upon thousands of people are
dying as a result of its ravages.

DISEASES CAUSED BY PARASITES

Some two hundred years ago, after it became known that minute animal
parasites were associated with certain diseases and were the cause of
them, it rapidly came to be believed that all our ills were in some way
caused by such parasites, known or unknown. Further study and
investigation failed to reveal the intruders in many instances and so it
began to be doubted whether after all they were responsible for much
that had been laid at their doors. Then after it was discovered that
minute plant parasites, bacteria, were responsible for many diseases
they in turn began to be accused of being the cause of most of the ills
that the flesh is heir to.

In later years we have come to adopt what seems to be a more reasonable
view, for we can see and definitely prove that neither of these extreme
views was correct but that there was much truth in each of them. To-day
we recognize that certain diseases,