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 lurking in the soil. Leprosy is communicated from a leper in the same way. The almost ubiquitous bacteria of blood-poisoning (septicaemia) may enter by the smallest fissure of the skin, still more readily by large cuts or wounds. The bites and stabs of small and large animals--wolves, dogs, flies, gnats, fleas and bugs, also open the way, and often the deadly microbe has associated itself with the biting animal and is carried by it, ready to effect an entrance. Thus rabies (hydrophobia) is introduced by the bites of wolves and dogs, and a whole series of diseases, such as plague, malaria, sleeping-sickness, gaol-fever (typhus), yellow fever, relapsing fever, and others, are introduced into the human body by blood-sucking insects. Hence the immense importance of treating every slightest wound and scratch with chemicals (called "antiseptics"), which at once destroy the invading microbe--and of keeping a wounded surface covered and protected from their approach. In ways at one time unsuspected, such openings may be made by which poisonous microbes enter the body. Thus the little hard-skinned parasitic thread-worms which are often brought in by uncooked food into man's intestine, though by themselves comparatively harmless, scratch the soft lining of the bowel and enable poison-making microbes