Page:Popular Science Monthly Volume 34.djvu/240

228 with the density of the population in the districts considered, until with a population of 65,823 to a square mile, the death-rate for all ages reaches 38·67 per 1,000, and for those under five, 139·52 per 1,000—that is, while the total death-rate is increased a little more than twofold, that under five was nearly-quadrupled.

Another fruitful cause of death, and one which especially helps to swell the mortality of infancy, is contagion, including in that term all those influences by which disease is communicated from one individual to another, either by direct contact or through the atmosphere. This is the prime cause of the large class of zymotic or germ diseases, which in their various forms carry off nearly one third of all children dying under the age of five years. In general terms it may be stated that, while contagion is the direct cause of these diseases, yet their prevalence and fatality are in inverse proportion to the general observance of the laws of public sanitation and private hygiene.

The depressing influences of extreme poverty, filth in all its forms, and the overcrowding of large cities, are great promoters of contagion, resulting in epidemics, plagues, and pestilences; while strict cleanliness, fresh air, pure water, and hygienic living, tend greatly to restrict its spread and prevent these results. Temperature, also, has much to do with the prevalence of zymotic diseases, some of which require a certain high degree of average temperature, while others thrive best in cold weather. Extreme degrees of heat and cold (boiling and freezing) destroy the life of most germs, but not of all. Thus, the first sharp frosts of autumn cut short the progress of yellow fever, while diphtheria is somewhat more prevalent in winter than in summer. The strict isolation of the sick, and careful disinfection of their surroundings, are also essential to the limitation or prevention of contagious diseases.

The death-rate among infants and young children is especially influenced by the five principal acute contagious or infectious diseases—namely, measles, scarlet fever, small-pox, diphtheria, and whooping-cough. According to English life-tables, these five diseases were the cause of 18·8 per cent of the entire mortality under five for the ten years from 1860 to 1870, while the average age of all persons dying from whooping-cough was 1·8 years, from measles 2·7 years, from diphtheria and scarlet fever 5·8 years, and from small-pox 11·9 years.

This latter disease, which was formerly by far the most fatal of the class, has of late years been shorn of its terrors by the beneficent discovery of Jenner. Thus, during the sixteenth and seventeenth centuries, it is estimated that from seven to twelve per cent of all deaths were caused by small-pox; while since the