as with the hookworm, while eggs are produced in the
human body, they have no directly detrimental effect, the objectionable
feature of their residence being due to the fact that the continual
draught which they make upon the blood vessels of the intestine reduces
the vitality, causing anaemia.

In other cases, as with the guinea worm, found in Africa and South
America, the worm wanders from the stomach, which it enters toward the
surface of the body, and finally breaks through, causing ulcers or
abscesses.

In still other cases, as with that form of filaria which causes
elephantiasis, the adult worm or the embryos are present in the
lymphatics in such numbers as to interfere with circulation, causing the
fearful swellings characteristic of the disease named.

Finally, in such cases as trichinosis and tapeworm, there is usually but
little inconvenience to the human being harboring them, except when
their number becomes very large. Then there may be diarrhoea, loss of
appetite, and other digestive disturbances. The different tapeworms are
generally responsible for nothing more than indigestion and nervousness.
These latter parasites are, however, formidable in so far as their size
is concerned. The mature pork tapeworm is about ten feet long, although
the eggs, seen in the pork flesh, giving it its name of "measly," are
only about a thousandth of an inch in diameter. The fish tapeworm, when
mature, measures about twenty-five feet in length, while the beef
tapeworm is about the same length. These worms can develop only in the
bodies of the animals named, and find their way into the human body only
through the medium of imperfectly cooked meat.

If proper precautions be taken in these directions, if only water is
used for drinking which is known to be free from such parasites and
their eggs, and if insects like mosquitoes and fleas are kept away by
screening windows and doors, and if meat be always thoroughly cooked,
the dangers of diseases from parasites will be reduced to a minimum.

_Bacterial agencies._

By far the most important of the living agencies concerned with the
direct production of disease are those small vegetable organisms known
as bacteria. Not all bacteria, by any means, produce disease; in fact,
it is not too much to say that the majority of bacteria are benefactors
to the human race. Their chief agency is not to cause disease, but to
prevent it, and they do this because they are able to transform