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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    The epidemiological problem is twofold. In its one aspect it concerns the distribution of epidemic disease in space. That is to say, if an infectious disease be introduced into the midst of a uniformly populated district, its subsequent distribution necessarily follows some definite law. Practically a complete mathematical solution of this part of the problem has been given by Professor Pearson, and his result only requires modification to meet particular cases. It is purely an applica- tion of the theory of chance. The second part of the problem concerns the distribution of epidemics in time; that is, the manner in which the number of new cases varies from day to day, and it concerns the laws which regulate the ascent and descent of the epidemic curve. This is not a problem in pure chance; it involves a knowledge of the rate at which certain things vary. It must be obvious that one of three things causes an epidemic to die out. Firstly, the conclusion of an epidemic may be due to the exhaustion of susceptible persons among the population. In the second and third place, either a loss of infectivity on the part of the organism or of susceptibility on the part of the population is necessary. This part of the subject is bio- logical and independent of the theory of chance, and will first be considered. There are two ways in which the biological basis of epidemics may be ascertained : one is by observation of the mode of epidemic progress, either as it occurs in nature or in experiment ; the other is by examination of the accumulated statistical information at our disposal. The latter at present lends itself much more easily to dis- cussion. The fact which presents itself at once to our notice is the near symmetry of the form of the epidemic curve. If any typical epidemic of plague, as at present in India, or in former times—say that of 1665 in London—is examined, it is seen at once that as the disease increased, so it declined. That is even more fully seen in those directly infectious cases, such as small-pox and measles. The last large epidemic of small-pox in London in 1901-2 is a case in point. In fact, take any large solitary epidemic (by this phrase I mean an epidemic in which the level from which the disease rises is only a minute fraction of the epidemic height), and the symmetry of the course is an obvious and well-marked feature. The deduction from this phenomenon is direct and complete—namely, that the want of persons liable to infection is not the cause of the decay of the epidemic. On no law of infection which I have been able to devise would such a cause permit of epidemic symmetry. The fall must in