er set of disease-causing protozoan parasites
which are similar to the amoeba or proteus-animalcule, and a third,
which belong to the group of "ciliated infusoria." They are not so
minute as the preceding set, and are not usually referred to as
"microbes." They inhabit the intestine of man and animals, and cause,
in some instances, dysentery. These two later kinds of protozoan
parasites I will at the moment leave out of consideration, as well as
the "coccidia," which multiply in the tissue-cells of animals--for
instance, rabbits and mice--and cause an unhealthy growth and
excessive multiplication of the cells of the tissues, which in some
respects resembles that seen in the terrible disease known as cancer.
Indeed, it is held by many investigators that some such
parasite--though not yet discovered--is the cause of cancer.

A very important question is: How do these poison-producing parasites
(for it is by the poison which they manufacture that they upset the
healthy life of their hosts) make their way into the human body? The
surface of the body of animals, like man, is protected by a delicate,
horny covering--the epidermis--through which none of these parasites
can make their way. They can only get through it, and so into the
soft, juicy tissues and the fine blood-vessels which it covers, when
it is cracked, broken, pierced, or cut. But they also have a way to
open them through the softer moist surfaces of the inner passages,
such as the digestive canal and the lungs. They enter (some kinds only
and not a few) with food and drink into the digestive canal, and with
the air into the air-passages and the lungs; and once in these
chambers, which have only soft lining-surfaces, they are able to
penetrate into the substance of the body. Many of those which enter
the digestive canal do not require to penetrate further, but multiply
excessively in the contents of the bowel, and there produce poisons,
which are absorbed and produce deadly results--such are the bacteria
which produce Indian cholera and ordinary diarrhoea--whilst the kind
causing typhoid fever not only multiplies in the gut, but penetrates
its surface.

The protective surface of man's body is broken, and the way laid open
for the entrance of microbes in various ways. A slight scratch,
abrasion, or even "chapping" is enough. Thus, a mere breaking of the
skin of the knuckles by a fall on to dirty ground lets in the deadly
bacterium of lock-jaw (tetanus), which is