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GENERAL REMARKS UPON INFECTION
M UCH confusion exists in the popular use of the
term “ infection.” By many it is supposed to be synonymous with contagion. But this is not in accordance with established usage among well-informed physicians. It is a far more comprehensive term ; for, while all contagious diseases are infectious, not all infectious diseases are contagious. A contagious disease is one which may be transmitted by personal contact, as, for example, smallpox, measles, scarlet fever. These diseases may also be communicated, indirectly, through the medium of objects which have been in contact with infected individuals, such as clothing, bedding, etc. (“fomites "). While the contagious diseases mentioned and others which are generally recognised as liable to be contracted by contact with the sick are also infectious, there are numerous diseases which are infectious but not contagious. As examples of this class we may mention the malarial
fevers, yellow fever, and trichinosis. Still others, such as cholera, typhoid fever, and dysentery are only transmitted by personal contact under very exceptional circumstances.
The word “infection,” from the Latin verb inficio, means literally to put or dip into anything; but in its accepted technical sense the putting of non-living particles into a living body does not constitute infection. A man who receives a load of bird-shot in the muscles of his thigh is not infected with bird-shot or with lead. But when a living micro-organism is introduced into the body of a living animal and multiplies there, the animal is infected. This infection may be localised, as in the case of a carbuncle, an abscess, a pneumonia, a pleurisy, etc., or it may be a general blood infection, as in relapsing fever, yellow fever, or the malarial fevers. In cholera and dysentery the infectious agent is in the alimentary canal, and penetrates to a greater or less extent the mucous membrane of the intestine. In typhoid fever it invades the glands of the intestine and mesentery, and the spleen. In diphtheria the mucous membrane of the throat and posterior nares is the usual seat of the infection.
Certain infectious diseases have their seat in various favourite localities upon the external portion of the body. These are the infectious skin diseases,
which are also contagious for evident reasons. The fact that the infecting parasite must penetrate the body of its living host is well illustrated by the infectious skin disease known popularly as “itch” (scabies). The itch insect deposits its ova and rears its family in burrows beneath the epidermis, and thus becomes the agent in the production of an infectious skin disease. Certain other parasites, known as pediculi, infest the surface of the body, especially in localities covered with hair. As these do not penetrate the skin, their presence does not constitute an infection.
The fact that a disease may be transmitted through a series of individuals either by contagion or in some other way—by inoculation, by contaminated drinking-water, etc.—is evidence that it is due to a living disease germ of some kind and that it is consequently infectious. An individual who has been stung by a wasp or bitten by a rattlesnake is not infected but poisoned. The symptoms resulting from such a bite cannot be reproduced in another individual by inoculation of blood or other material from the body of the person bitten. But the bite of a rabid dog gives rise to an infectious disease—“hydrophobia "—which may be transmitted by inoculation through a series of men or dogs or other susceptible animals.
The poison introduced by a wasp or rattlesnake
does not multiply in the body of the individual bitten and the symptoms produced bear a direct relation to the amount injected. Moreover, the symptoms follow very closely after the bite. On the other hand, an infectious disease resulting from inoculation, or contracted in any other way, is not developed at once; but after the introduction of the infectious material a certain interval elapses, technically known as the “period of incubation,” before the symptoms characteristic of the disease are manifested. In the case of hydrophobia, resulting from the bite of a rabid animal, this so-called period of incubation may be greatly prolonged. It is seldom less than two weeks and may be six months or more. But, as a rule, the period of incubation is quite definite for each infectious disease, although differing greatly in different diseases. Thus it is usually less than three days in scarlet fever and diphtheria; from two to five days in yellow fever and influenza; from seven to ten days in whoopingcough; eleven or twelve days in smallpox; fourteen days in measles; and from seventeen to twenty-one days in mumps. In wound infections, resulting from the introduction of certain well-known disease germs into wounds produced by the surgeon's knife or otherwise, the period of incubation is comparatively short, and erysipelas or“ blood-poisoning” may be developed within a few hours after the inoculation occurs.
What has already been said will, it, is hoped, make the following definition of an infectious disease quite clear. An infectious disease is one which is caused by the introduction of living disease germs within the body of a susceptible individual. This definition includes the idea of the reproduction—that is, multiplication within the body—of the specific disease germ, which must be living and thus capable of reproduction in order to produce an infectious disease. Moreover, it must find conditions favourable for its reproduction within the body or it will not give rise to any disease process—that is, the individual must be susceptible.