The factors that determine the rise, spread, and degree of severity of epidemic diseases / M. Greenwood.

  • Greenwood, Major, 1880-1949.
Date:
1913
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Licence: In copyright

Credit: The factors that determine the rise, spread, and degree of severity of epidemic diseases / M. Greenwood. 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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    we fall into the error of Graves’s teachers in the early nineteenth century. Some have attempted to account for the facts on the basis of an appeal to natural selection. Scarlet fever was once widely prevalent, and it is possible that absolute or relative immunity depends upon innate heritable characters. Consequently the proportion of insusceptibles in a population tends to increase. If, however, the causative agent be an obligatory parasite, a diminution in the number of susceptibles necessarily involves a diminution in the number of opportunities to become infected. This, in its turn, leads to a slackening in the intensity of selection; innate immunity ceases to be a character of survival value and the way is paved for another formidable outbreak if only the disease have not become really extinct. We might suppose that the immunity acquired by the selective action of a series of bad epidemics is gradually lost, and that in a few generations the soil is ready for a fresh crop of the disease. It is, of course, true that nothing in the ascertained history of scarlet fever disproves this hypothesis, but if one considers the vicissitudes of such a disease as plague through a very long period, as narrated with such wealth of illustration and lucidity by Professor Sticker, we shall have doubts as to the sufficiency of so facile an interpretation. We must not confine our application of the selection hypothesis to the human subject of the disease. Much work, in particular the researches of Penfold, Muller, and other bacteriologists, has demonstrated that mutations or even fluctuating variations may lead to the evolution of bacterial species, or perhaps, more accurately speaking, to the production of varieties differing profoundly from the parent stock in physiological properties. The application of these results to the phenomena of secular variations of epidcmicity is a matter for the future in the case of a disease like scarlet fever, the materies morbi of which elude our actual powers of identification. If we apply the selection hypothesis to the morbific organisms them- selves, it seems clear that any obligatory parasite must either submit to modifications of its virulence, acquire powers of enormous multiplication, or perish in the struggle for existence. Here again we must remember the complexity of the problem. It is not only changes in the essential materies morbi of scarlet fever which are in question, but also variations in the secondary invaders which are so often responsible for the graver complications of the disease. In this connexion we should have to con- sider variations and mutations of myriads of pathogenic streptococci and require to take account of bacteriological phenomena which I am not in a position even to enumerate. In an interesting paper by Nash and the comments thereon of the late Dr. Payne, the conception of evolutionary change among competing organisms of disease is traced out in some detail. It would exceed the functions of a reporter were I to attempt a critical examination of such work. Ihc evidence as yet produced is hardly sufficient to enable