CARRIERS OF DISEASE

It has now been discovered that a great number of human diseases are caused by microscopic parasites, which are spoken of in a general way by the name invented by the great Pasteur, viz. "microbes." Wool-sorter's disease, Eastern relapsing fever, lock-jaw, glanders, leprosy, phthisis, diphtheria, cholera, Oriental plague, typhoid fever, Malta fever, septic poisoning and gangrene have been shown to be caused each by a peculiar species of the excessively minute parasitic vegetables known as bacteria (or Schizophyta). Others, for example, malaria and sleeping sickness, have been shown to be caused by almost equally minute microbes, which are of an animal nature, and similar to the free-living animalcules which we call Protozoa, or "simplest animals," whilst a third lot of diseases—rabies, smallpox, yellow fever, scarlet fever, and typhus—are held to be caused by similar minute parasites, although these have not yet actually been seen and cultivated, but are surely inferred (from the nature and spread of these diseases) to exist.

The difference of the microbes called bacteria from the disease-causing microbes classed as "Protozoa" consists in their simpler structure and mode of growth. They are essentially filaments which continually multiply by fission—a process often carried so far that the little organisms present themselves as short rods, or as curved (comma-shaped), or even spherical particles (micrococci)—and only in favourable conditions arrest their self-division so as to grow for a time into the thread-like or filament shape. Often these filaments are not straight, but spirally twisted, and are called "spirilla." Some of them are blood parasites, but the larger number attack the tissues, and others occur in the digestive canal.

The parasitic disease-producing protozoa, on the other hand, are of softer substance, often have the habit of twisting themselves in a corkscrew-like manner, and usually are provided with an undulating membrane or frill, as well as with one or with two whip-like swimming processes (the latter are present also and are often numerous in the actively swimming phases of bacteria), and have a more complicated life-history. They divide, as a rule, longitudinally and not transversely, and pass from one "host" to a second, where they assume distinct forms—males and females, which conjugate and break up (each conjugated or fused pair) into a mass of very numerous, excessively minute, young. The disease-producing protozoa of this kind are frequently parasitic in the blood of man and animals, and were only recently recognised, after the disease-producing bacteria of many kinds had been thoroughly studied. These animal microbes are often spoken of as "blood-flagellates" or hÆmo-flagellata, and the larger kinds are called "Trypanosomes," or "screw-form parasites," or whilst a series of more minute ones are called "Piroplasma," or "pear-shaped parasites." Many, but not all, are found during a certain period of their life, actually inside the corpuscles of the blood. The fact that many of these blood-flagellates (if not all) have, besides their life in the blood of one species of animal, a second period of existence in the juices or the gut of another animal, has made it very difficult to trace their migrations, since in the second phase of their history their appearance differs considerably from that which they presented in the first. And often they exist in one kind of animal without doing any harm, and are only poisonous when introduced by insects into the blood of other kinds of animals!

There is, further, another 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 lurking in the soil. Leprosy is communicated from a leper in the same way. The almost ubiquitous bacteria of blood-poisoning (septicÆmia) 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 to enter the deeper tissues, and cause dangerous abscesses and appendicitis.

The carriers of disease germs thus become a very important subject of study. There are carriers which make no selection, but are, so to speak, "casual" in their proceedings, and there are others which have the most special and elaborate relations to some one kind of disease-causing microbe for which alone they are responsible, and to the life of which they are necessary. Let us look first at the more casual group. Man himself is a great carrier and distributor of his own diseases. Unless and until he has learned to be careful and guard against thoughtless proceedings, he is always spreading the microbes of his diseases and passing them on to his fellow men. He pollutes the waters, rivers, lakes, and pools from which others drink. He manures his crops, and then eats some of them uncooked. His hands are polluted by disease-causing microbes, and he handles (to an alarming and unnecessary extent) the food, such as bread and fruit, which is swallowed by his fellows, without cleansing it by heat. It has lately been shown that apparently healthy men and women often harbour within them the microbes of typhoid fever or of cholera (and probably other diseases), without themselves suffering in health, and that unsuspected they thus become distributing centres of these diseases. The names "typhoid carrier" and "cholera carrier" have actually been introduced to describe the condition of such persons. Then, again, by his breath, and by coughing and spitting, a man acts as a carrier to others of disease-microbes already lodged in him, as well as by actual contact in the case of those infections which are called "contagious." The numerous animals which surround and are associated with man act very largely as casual carriers and distributors of disease microbes. Thus dogs and even the cleanly cat are frequently carriers of disease. But more especially those creatures which visit man's food stores and food ready for consumption (such as bread, fruits, cold meat, etc.) are active carriers. Rats and mice run over such stores and pollute them. But the most widely active in this way is the common house-fly.

Whilst white men have developed an almost automatic resistance and objection to the visits of flies to their lips, eyelids, and any wound or scratch of the skin—a resistance which is not shown by many savage races—they yet allow house-flies to swarm in their dwellings, to run about and sample their food, with an indifference which is, when the truth is known, truly horrible in its fatuity and foolhardiness. For the fact is that the feet and proboscis of the common house-fly are covered with microbes of all sorts, picked up by his explorations upon every kind of filth. At every step which he takes he plants a few dozen microbes, which include those of infantile diarrhoea, typhoid, and other prevalent diseases. This is easily shown by allowing him to walk over a smooth plate of sterilised nutritive gelatine and preserving it afterwards free from the access of microbes from the air. In twenty-four hours every footstep of the fly on the gelatine is marked by an abundant and varied crop of microbes, which have multiplied from the individuals let drop by the little pedestrian. There is no doubt whatever that the house-fly is a main source of the dissemination of the microbe of infantile diarrhoea, and the cause annually of hundreds of thousands of deaths of children in the great cities of Europe and America. Also in camps and infected districts he is largely responsible for the introduction of the microbe of typhoid fever into the human food to which he has free access after his previous visits to open latrines. The house-fly is himself a product of dirt and neglect. The eggs are laid in old manure heaps and kitchen middens, and the maggots, which eventually are transformed into flies, nourish themselves in those accumulations. When this refuse is rapidly and regularly removed by the care of the sanitary officials of a town, the flies diminish in number, as they have diminished in London within the last thirty years. We no longer are overrun by flies in London in the summer months. The man selling sheets of sticky paper is no longer heard in our streets calling "Catch 'em alive, oh!" But in country places, where a neglected stable-yard is near the dining room of the inn, house-flies are as great a nuisance and danger as ever. There is no difficulty, if the simplest rules of cleanliness are observed, in abolishing them altogether from human association, but combined and simultaneous action against them is an essential condition of success.