Disease and Its Causes by William Thomas Councilman (early reader books .TXT) 📕

Although the algæ are useful guests, when they become too numerous the protozoan devours them. It is evident that symbiosis is the most favorable condition for the existence of the parasite, and an injurious action exerted by the parasite on the host unfavorable. The death of the host is an unfortunate incident from the parasite's point of view in that it is deprived of habitation and food supply, being placed in the same unfortunate situation as may befall a social parasite by the death of his host. Chapter VII

The Nature Of Infection.—The Invasion Of The Body From Its Surfaces.—The Protection Of These Surfaces.—Can Bacteria Pass Through An Uninjured Surface.—Infection From Wounds.—The Wounds In Modern Warfare Less Prone To Infection.—The Relation Of Tetanus To Wounds Caused By The Toy Pistol.—The Primary Focus Or Atrium Of Infection.—The Dissemination Of Bacteria In The Body.—The Different Degrees Of Resistance To Bacteria Shown By The Various Organs.—Mode Of Action Of Bacteria.—Toxin Production.—The Resistance Of The Body To Bacteria.—Conflict Between Parasite And Host.—On Both Sides Means Of Offense And Defense.—Phagocytosis.—The Destruction Of Bacteria By The Blood.—The Toxic Bacterial Diseases.—Toxin And Antitoxin.—Immunity.—The Theory Of Ehrlich.

As has been said, infection consists in the injury of the body by living organisms which enter it. The body is in relation to the external world by its surfaces only, and organisms must enter it by some one of these surfaces. It is true that the bacteria in the intestine—either those normally present or unusual varieties—may, under certain circumstances, produce substances which are injurious when absorbed; but this is not infection, and is analogous to any other sort of poisoning. Each surface of the body has its own bacterial flora. Organisms live on the surface either on matter which is secreted by the surface or they use up an inappreciable amount of body material. Many of these bacteria are harmless, some are protective, producing by their growth such changes in the surface fluids that these become hostile to the existence of other and pathogenic forms. The surfaces also frequently harbor pathogenic organisms which await some condition to arise which will permit them to effect entrance into the tissues.

The surfaces of the body protect from invasion to a greater or less degree. The skin protects by the impervious horny layer on the outside, the external cells of which are dead and constantly being thrown off. Bacteria are always found on and in this layer, but the conditions for growth here are not very favorable and the surface is constantly cleansed by desquamation. The new cells to supply the loss are produced in the deepest layer of the epidermis, and the movement of cells and fluids takes place from within outwards. The protection is less perfect about the hairs and the sweat glands. Infection by the route of the sweat glands is, however, uncommon, for the sweat is a fluid unfavorable for bacterial growth and the flow acts mechanically in washing away organisms which may have entered the ducts. Infection by the route of the hair follicles is common. There is no mechanical cleansing as by the sweat, the space around the hair is large and the accumulated secretion of the hair glands and the desquamated cells furnish a material in which bacteria may grow. Growing as a mass in this situation, they may produce sufficient toxic material to destroy adjacent living cells and thus effect entrance. Infection from the eye is not common, the surface, though moist, is smooth; the eyelashes around the margin of the lids give some mechanical protection from the entrance of bacteria contained in dust, and the movements of the lids and the constant and easily accelerated secretion of tears act mechanically in removing foreign substances. It is possible that the mechanical cleansing of the skin by the daily bath may have some action in preventing infection.

The internal surfaces are much more exposed to attack and the protection is not so efficient. The moisture of these surfaces is both a protection and a source of danger. It protects by favoring the lodgment near the orifices of organisms which are in the inspired air, for when bacteria touch a moist surface they cannot be raised from this and carried further by air currents. The moisture is a source of danger in that it favors the growth of bacteria which lodge on the surface. The respiratory surface which is most exposed to infection from the air is further protected by the cilia, which are fine hair-like processes covering the cells of the surface and which by their constant motion sweep out fine particles of all sorts which lodge upon them. The cavity of the mouth harbors large numbers of organisms, many of them pathogenic. It forms a depot from which bacteria may pass to communicating surfaces and infection from these may result. Food particles collect in the mouth and provide culture material, and there are many crypts and irregularities of surface which oppose mechanical cleaning. Infection of the middle ear, the most common cause of deafness, takes place by means of the Eustachian tube which connects the cavity of the ear with the mouth. Organisms from the mouth can extend into the various large salivary glands by means of the ducts and give rise to infections. The tonsils, particularly in children, provide a favorable surface for infection. The mucous surface extends into these forming deep pockets lined with very thin epithelium, and in these débris of all sorts accumulates and provides material favorable for bacterial growth.

The lungs at first sight seem to offer the most favorable surface for infection. The surface, ninety-seven square yards, is enormous; it is moist, the epithelial covering is so thin as to give practically no mechanical protection, large amounts of air constantly pass in and out, and the surface is in contact with this. They are protected from infection in many ways. The tubes or bronchi by which the air passes into and from the lungs are covered with cilia; the surface area of these tubes constantly enlarges as they branch, the sum of the diameters of the small tubes being many times greater than that of the windpipe, and this enlargement by retarding the motion of the air favors the lodgment of particles on the surface whence they are removed by the action of the cilia. The entering air is also brought closely in contact with a moist surface at the narrow opening of the larynx. That bacteria and other foreign substances can enter the lungs in spite of these guards is shown not only by the infections which take place here, but also by the large amount of black carbon deposited in them from the soot contained in the air.

Infection rarely takes place from the surface of the gullet or oesophagus which leads from the mouth to the stomach. This is due to the smoothness of the surface and to the rapidity with which food passes over it. Infection by the stomach also is rare, for this contains a strong acid secretion which destroys many of the bacteria which are taken in with the food. It is found impossible to infect animals with cholera unless the acidity of the stomach contents be neutralized by an alkali. Many organisms, although their growth in the stomach is inhibited, are not destroyed there and pass into the intestines, where the conditions for infection are more favorable. This large and very irregular surface is bathed in fluid which is a good culture medium and but a single layer of cells covers it. The organisms which cause many of the infectious diseases in both man and animals find entrance by means of the alimentary canal, as cholera, dysentery, typhoid fever, chicken cholera, hog cholera.

Infection by the genito-urinary surface is comparatively rare. The surface openings are usually closed, and the discharge of urine has a mechanical cleansing effect. The wide tube of the vagina is further protected by a normal bacterial flora which produces conditions hostile to other and pathogenic bacteria. The most common infections are the sexual diseases, which are due to organisms which find favorable conditions for growth in and on the surface and which are conveyed from a similar surface by sexual contact.

It remains a question whether bacteria can penetrate an intact surface producing no injury at the point of entrance and be carried by the lymph or blood into internal organs where they produce disease. Internal infections are often found with seemingly intact body surfaces, but it is impossible to exclude the presence of minute or microscopic surface injuries by which the organisms may have entered. It is also possible that a slight injury at the point of entrance may heal so completely as to leave no trace.

The chief danger from wounds is that their surfaces may become infected. Death from wounds is due more frequently to infection than to the actual injury represented by the wounds. Much depends upon the character of the wound. Infection of clean wounds which are made by a sharp cutting instrument and from which there is abundant hæmorrhage with sealing of the edges of the wound by clotted blood, rarely happens. Typical wounds of this sort are often made in shaving, and infection of such wounds is extraordinarily rare. If, with the wound, pathogenic organisms are placed in the tissue, or foreign substances such as bits of clothing are carried in with a bullet, for example, or if the instrument causing the wound be of such a character as to produce extensive lacerations of tissue, infection is more apt to occur. The less frequency of infection in modern wars is in part due to the simpler character of the wounds and in part to the fact that modern fixed ammunition is practically free from germs. The old spear-head, the arrow, the cross bow bolt, had little regard for the probabilities of infection. Whether infection follows a wound depends both upon the entry of pathogenic organisms and upon these finding in the tissues suitable opportunities for growth. In wounds in which there is much laceration of tissue organisms find the most favorable conditions for development. The very slight wounds produced by the exploded cap in the toy pistol give suitable conditions for the development of the bacillus which produces tetanus or lockjaw. The deaths of children from lockjaw following a Fourth of July celebration have often exceeded the total deaths in a Central American revolution. The tetanus bacillus is a widely distributed organism, whose normal habitat is in the soil and which is usually present on the dirty hands of little boys. The toy-pistol wounds are made by small bits of paper or metal being driven into the skin by the explosion of the cap. The wound is of little moment, the surface becomes closed, and a bit of foreign substance, a few dead cells and the tetanus bacilli from the surface remain enclosed and in a few days the fatal disease develops. Infection of the surfaces of old wounds such as the surface of an ulcer takes place with difficulty. Large numbers of leucocytes which give protection by phagocytosis are constantly passing to the surface, and there is also a constant stream of fluid towards the surface. On such a surface there may be an abundant growth of pathogenic organisms, but no infection results.

In most infections there is a focus where the infectious organisms are localized; this may correspond to the point of entrance on a surface or it may be in the interior of the body, the organisms being deposited there after entrance. At this primary localization, the atrium of infection,9 the organisms multiply and from this point further invasion takes place. Many secondary foci may be formed in the organs by distribution of the organisms, or there may be infection of the blood and fluids of the body. The injuries which are produced depend upon the nature of the infecting organisms. The most common lesion consists in the death