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disease germs may find their way from the excreta of the sick to the stomachs of healthy persons. If thrown upon the ground, flies alighting upon the foul material may subsequently visit a near-by kitchen and there walk over the food prepared for the family meal, leaving numerous typhoid bacilli in their tracks; or they may fall into the milk, or in some other unsuspected way convey the deadly microscopic germs to some article of food or drink. Again, articles of clothing soiled by the discharges of the sick may be the means of conveying infection to laundresses, who in handling such articles are liable to soil their hands or in some indirect way to introduce the pathogenic bacteria into their mouths. This mode of infection is liable to occur in any disease in which the germ is present in the discharges from the bowels, and especially in Asiatic cholera.
In Oriental countries, where human excreta constitutes a very common fertilising material, green vegetables, which are eaten raw, are believed to serve as the medium through which the germs of dysentery and cholera are occasionally conveyed to the stomachs of persons partaking of such articles of food. There is also considerable evidence in favour of the view that typhoid fever may be contracted by eating oysters which have been grown in sewage-polluted waters. The infectious disease known
as trichinosis is contracted by eating pork containing living trichinæ. As these parasitic worms are promptly killed at a comparatively low temperature, cooked pork is quite harmless, so far as this disease is concerned.
Infection through wounds is far less common at the present day than was the case before aseptic surgery and the antiseptic treatment of wounds became established as a standard method of surgical procedure. Formerly epidemics of septicæmia, erysipelas, and hospital gangrene were of frequent occurrence, and the cleanest and best regulated hospitals were not exempt from these visitations. But in the light of our present knowledge such epidemics are no longer excusable and the infection of surgical wounds is extremely rare. Accidental wounds may, however, become infected at the time they are inflicted or because of failure to apply proper surgical dressings. Jagged and penetrating wounds which do not bleed are especially liable to be infected by the lodgment of germs in the deeper portion of the wound. It is in this way that tetanus or lockjaw is commonly produced. The tetanus bacillus forms spores which may retain their vitality for years. These are found in soil which has been enriched by manuring and in the dust of streets.
Bubonic plague is another disease which is com
monly contracted through accidental wounds. In countries where it prevails, it has been observed that the natives, who do not wear shoes and stockings, are much more liable to infection than Europeans, and it seems to be well established that infection may occur through insignificant wounds, such as scratches or abrasions of exposed parts of the body. We have also satisfactory evidence that tuberculosis may be transmitted to man by the accidental inoculation of an open wound. Malignant pustule, or anthrax, is communicated in the same way, and it sometimes happens that the inoculation is effected by flies which have been in contact with the infectious material escaping from the body of an animal having the disease or recently dead as a result of it.
It is well known that surgeons when operating upon an infected wound and pathologists when performing autopsies, in certain cases, are liable to a severe and sometimes fatal attack of “ blood-poisoning” as a result of infection through a slight scratch or abrasion upon the hand, or through an accidental puncture made by a surgical needle. The germ which is most frequently concerned in this blood-poisoning, or septicæmia, is well known and is the usual cause of puerperal fever, erysipelas, and a considerable proportion of the cases of peritonitis. It is, therefore, in treating or inaking autopsies upon cases of this nature
that physicians run the greatest danger of accidental infection.
The question whether infection may occur through the unbroken skin has been studied and an affirmative result obtained. The liability to infection in this way is, however, comparatively slight. When it does occur, it appears that the germs penetrate through the hair follicles. Infection may occur through mucous membranes, and it is in this way that infectious conjunctivitis and various other specific inflammations of mucous membranes are propagated. Infectious skin diseases, such as scabies (itch), ringworm, barber's itch, etc., may, no doubt, be contracted by susceptible persons, when conditions are favourable, independently of any wound or abrasion, especially in those who do not indulge in frequent bathing and thus give the germs time to penetrate the epidermis.
Researches made during the past few years have demonstrated that malarial fevers and yellow fever are communicated to man through the bites of infected mosquitoes. Certain infectious diseases of lower animals are also transmitted by insects. Thus it has been shown that ticks are responsible for the propagation of a fatal disease of cattle known as Texas fever, and an infectious disease of horses, which has recently prevailed extensively in the Philippine Islands, is communicated by a biting fly, which
transmits the parasite from diseased to healthy animals —“surra disease.” The tsetse-fly disease of Africa is transmitted in the same way and is very fatal to horses and also to the ox, the dog, the ass, and the sheep, but not to wild animals indigenous in the region where the tsetse fly is found. The parasite is present in great numbers in the blood of infected animals, and the fly simply acts as a carrier of this parasite from diseased to healthy animals.