course a most fortunate
thing for us, for the parasitic germs are everywhere.

There is perhaps "more truth than poetry" in the following newspaper
jingle:

"Sing a song of microbes,
Dainty little things,
Eyes and ears and horns and tails,
Claws and fangs and stings.
Microbes in the carpet,
Microbes in the wall,
Microbes in the vestibule,
Microbes in the hall.
Microbes on my money,
Microbes in my hair,
Microbes on my meat and bread,
Microbes everywhere.
Microbes in the butter,
Microbes in the cheese,
Microbes on the knives and forks,
Microbes in the breeze.
Friends are little microbes,
Enemies are big,
Life among the microbes is--
Nothing '_infra dig_.'
Fussy little microbes,
Millions at a birth,
Make our flesh and blood and bones,
Keep us on the earth."

While of course most of these microbes are to be regarded as absolutely
harmless and some as very useful, many have the power to do much injury
if the proper conditions for their rapid development should at any time
exist. While the size of the parasite is always a factor in the damage
that it may do to the host the factor of numbers is perhaps of still
greater importance because of the power of very rapid multiplication
possessed by so many of the smaller forms.

Certain minute parasites in the blood may cause little or no
inconvenience, but should they begin to multiply too rapidly some of the
capillaries may be filled up and trouble thus result. Or take some of
the larger forms. A few intestinal worms may cause no appreciable effect
on the host, but as soon as their numbers increase serious conditions
may come about simply by the presence of the great masses in the host
even if they were not robbing it of its nourishment. Many instances are
known where such worms have formed masses that completely clogged up the
alimentary canal. Such injuries as these may be regarded as mechanical
injuries. Some parasites injure the host only when they are laying their
eggs or reproducing the young. These may clog up passages or some of
them may be carried to some more sensitive part of the body where the
damage is done. The guinea-worm of southwestern Asia and of Africa lives
in the body of its host for nearly a year sometimes attaining a great
length and migrating through the connective tissue to different parts of
the body causing no particular inconvenience until it is ready to lay
its egg