What is infection?
Much 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. While all infectious diseases are contagious, not all contagious diseases are infectious.
A contagious disease is one that may be transmitted by personal contact. Common examples are smallpox, measles, and scarlet fever. These diseases are directly transmitted through the medium of objects which have been in contact with infected individuals. The transmission medium may be clothing, beddings Etc.
While the contagious diseases mentioned and others that are generally recognized as liable to be contracted by contact with the sick are also infectious, there are numerous diseases that are infectious but not contagious.
Some examples of these are:
- Malarial fevers
- Yellow fever
- Trichinosis
Other diseases such as cholera, typhoid fever, and dysentery are only transmitted by personal contact under very exceptional circumstances.
The word infection, from the Latin word “inficio” means literally to put a dip into anything.
In its accepted technical sense the putting of non-living particles into a living body does not constitute infection. A man who receives birdshot in the muscles of his is not infected with birdshot or with lead.
When a living microorganism is introduced into the body of a living animal and multiplies there, the animal is infected.
This infection may be localized as in the case of a carbuncle, an abscess, pneumonia, pleurisy etcetera. It may sometimes be a general blood infection as in relapsing fever, yellow fever, or malarial fevers.
In cholera and dysentery, the Infectious agent is in the alimentary canal. It penetrates to a greater or lesser extent the mucous membrane of the intestine.
In typhoid fever, it invades the glands of the intestine, the mesentery, and the spleen.
In diphtheria, the mucous membrane of the throat and the posterior nerve is the usual seat of the infection.
Certain infectious diseases have their seat in various favorite localities upon the external portion of the body. These are infectious skin diseases that are also contagious for evident reasons.
The fact that the infecting parasite must penetrate the body of its living host is illustrated by the infectious skin disease known popularly as itch or scabies.
The itch insect deposits its ova, rears its family, burrows beneath the epidermis, and thus becomes the agent in the production of 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 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 b cannot be reproduced in another individual by inoculation of blood or other material from the body of the person bitten
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. The symptoms produced bear 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 symptom characteristics of the disease are manifested.
In the case of hydrophobia, resulting from the bite of a rabid animal, the period of incubation may be prolonged.
It takes two weeks in most cases but maybe six months or more. As a rule, the period of incubation is definite for each infectious disease although it happens differently for different diseases.
Incubation periods of common diseases.
- Scarlet fever and diphtheria-3 days
- Yellow fever and influenza-2-5 days
- Whooping cough-7-10 days
- Smallpox-11 to 12 days
- Measles-14 days
- Mumps-17-21 days
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.
An infectious disease can therefore be defined as one which is caused by the introduction of living disease germs within the body of a susceptible individual.
This definition includes the idea of reproduction, that is multiplication within the body of the specific disease germ which must be living and capable of reproduction in order to produce infectious disease.
The disease-causing germ must find conditions favorable for its reproduction within the body or it will not give rise to any disease process, that is, the individual must be susceptible.
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