Military hygiene / 2nd ed.

  • Caldwell, Robert, 1859-1919.
Date:
1910
Catalogue details

Licence: In copyright

Credit: Military hygiene / 2nd ed. Source: Wellcome Collection.

Provider: This material has been provided by Royal College of Physicians, London. The original may be consulted at Royal College of Physicians, London.

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    is that presented by vaccination. In this case, the disease of smallpox, by its passage through the cow, becomes so modi - fied that, as stated by McVail, the changes which take place in the body of the animal have the result of ' removing the objectionable and retaining only the valuable part of the disease.' It is not to be imagined that bacterial forms are mainly harmful; on the contrary, many of them, as will appear later, are essential to the continuance of life as we know it. Other forms, consisting of those found in the alimentary canal of man and the lower animals, are, as far as we know at present, not only perfectly harmless, but possibly fulfil useful purposes in the general scheme of Nature, although, in the latter connection, it must be admitted that present evidence points in the opposite direction. Having regard, however, to preceding Statements relative to certain changes in the virulence of disease-producing forms, it is reasonable to assume the possibility of a converse side to observed facts. In other words, is it, or is it not, possible for innocent forms to assume virulent powers under the influence of external conditions ?* Experience, as distinct from results obtained in a laboratory, answers this question in the affirmative. Experi- ments, no matter how ably conducted, can never have the same weight as conclusions which have been arrived at by Observation of the unaltered Operations of Naiure, and the latter may fairly claim priority of consideration over evidence of a purely negative character, obtained from a study of * ' Quite reccntly it has been found that there are non-pathogenic microbes of wicle distribution which, in every point except virulence, closely resemble the specific microbes of enteric fever and diphtheria, and non-pathogenic bacilH have been found to bc protective against anthrax. Some authorities regard thesc harmless microbes as being probably generically identical with the virulent microbes that they resemble, and therefore capable of assuming a virulent character if culti- vated under suitable conditions, a view that reintroduces the de novo hypolhesis in a new form' (Whitelegge, ' Hygiene and Public Health'). The significance of these facts as regards the present question is sufFiciently clear.