Certain considerations on the causation and course of epidemics / by John Brownlee.

  • Brownlee, John, 1868-1927.
Date:
1909
Catalogue details

Licence: In copyright

Credit: Certain considerations on the causation and course of epidemics / by John Brownlee. Source: Wellcome Collection.

Provider: This material has been provided by The University of Glasgow Library. The original may be consulted at The University of Glasgow Library.

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    all such cases be much more rapid than the rise, though, on the contrary, when asymmetry is markedly present the opposite holds. We are therefore left to explain the cessation of the disease on the ground of loss of infectivity on the part of the organism, or of a decrease in susceptibility on the part of the population. In either case the form of the epidemic curve allows the rate of this loss to be more or less accurately measured. Dr. Farr already gave the solution as far back as 1868, and his results were somewhat extended in a paper to the Epidemiological Society in 1874 by Dr. G. H. Evans. The form in which, however, Dr. Farr gives his arithmetical law of the epidemic does not allow of the underlying cause directly appear- ing. It remains a law without a reason. A different form of analysis is necessary to discover this. But the result is not difficult to ascertain. If, as he says, the second difference of the logarithms of the successive ordinates of the epidemic curves is constant, then it directly follows that the loss of infectivity of the organism is approximately in the ratio given by a geometrical progression. That is, if the infectivity of the organism of the epidemic is m, and at the end of a unit of time mg when g is less than unity, at the end of a second unit of time it will be mg2, at the end of the third mg3, and so on. Dr. Farr seems to have chiefly considered the matter from the point of pre- dicting the course of the epidemic, and there he is subject to special difficulties, for at best a law of this kind only expresses an average, and to make a prediction from the necessarily inexact figures obtainable at the beginning of an epidemic is somewhat futile. It may be taken, however, as certain that something approaching Dr. Farr’s law repre- sents the actual facts, and, with modifications to be mentioned presently, it can be made to describe the course of many epidemics with con- siderable accuracy. It is very difficult to give evidence at all conclusive as to whether susceptibility or infectivity plays the greater part in determining the course of epidemics, but to my mind the state of the organism as regards its power of infecting is much the more important. The explanation which makes an epidemic end because the whole suscep- tible population has passed through an attack of the disease is obviously out of count, but a few facts even on this point will perhaps be of interest. In the winter of 1907-8 one of the largest measles epidemics in the history of Glasgow occurred. Public health authorities can at present do very little in the way of limiting the spread of measles, so that artificial measures had practically nothing