Age incidence in zymotic diseases / by John Brownlee.

  • Brownlee, John, 1868-1927.
Date:
1904
Catalogue details

Licence: In copyright

Credit: Age incidence in zymotic diseases / 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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    instance is given by the age distribution of persons suffering from measles : the susceptibility to this disease decreases with each year of life from four years upwards, but on the curve of age distribution a marked secondary rise takes place between the ages of seventeen to thirty,^ which, when examined into, is found due to immigration to Glasgow of persons from country districts from which measles has long been absent, who consequently form a new population of susceptible persons at an age period at which among natives of Glasgow few such are to be found. It would seem that one of the cliief advantages offered by the application of the method in our case is to determine at what ages such causes are specially to be looked for. The method by which the appropriate probability curve can be found from the observations given in the statistics cannot be explained here. It depends in principle upon obtaining the curve belonging to the group of probability curves which has the same moments about a vertical line through its centre of inertia as has the curve given by observations. The first moment about a vertical line is that obtained by multiplying each veriical strip by its distance from the fixed line as in the process of finding the centre of the centre of gravity, the second by the square of that distance as in finding the “ moment of inertia,” the third by the cube of the distance, and the higher moments in the .same way. In general in infectious disease the origin of the curve must be placed at birth. With this assumption the number of moments required is reduced from four to three. It is true that in a few diseases abortion takes place, and some of these cases of premature birth may be due to the feetus having acquired the disease from which the mother is suffering. This especially applies to smallpox and to typhus fever, but with regard to all other infectious diseases such is either of great rarity or im- possible. The point of origin of the curves in general must thus be taken at the date of birth. Two classes of curves satisfy an asymmetrical distribution which begins at zero, rises to a maximum and then declines. In one of these the upper limit is definite and in the other indefinite. With most infectious diseases the susceptibility becomes so small at high ages as to be almost nil, and although it is more logical to assume a definite limit to life yet the simplification in calculation involved in leaving the upper