A glance at its etiology shows that it is a disease to which all things predispose that depress the general vitality: this is evidenced by the fact that children and old people are greatly depressed by the intense cold of winter and the chilling winds of March and April. Almost unknown in the polar regions, pneumonia is not an infrequent disease along the Mediterranean coast; and one peculiarity is to be noted here, that in cold as well as in warm climates moderate elevation above the sea-level predisposes to its occurrence.
Rainy seasons or moist districts do not influence the pneumonia-rate to any appreciable degree. Both these conditions have a direct effect in increasing the prevalence of bronchial catarrh, but they do not increase the pneumonia-rate.
The well-established facts that pneumonia occurs oftener among the poor than the wealthy; in the sailor when on shore oftener than when he is on shipboard; in soldiers oftener than among civilians at the same military post,—these are explained on the ground of better hygienic surroundings, better mode of life, nourishment, etc., of the one class as compared with the other. And in studying the predisposing causes of pneumonia one is led more and more to observe that it is the more liable to occur the less resistance individuals are able to offer to some (as yet unknown) specific pneumonic influence, and that depressing influences of whatever kind unquestionably predispose to croupous pneumonia.
The more dense the population in a district, the greater the pneumonia-rate. Hirsch says: "The amount of the mean fluctuation in the mortality from pneumonia is in inverse ratio to the density of the population." When a city has attained a certain size, wind, weather, seasons, and races have but a slight influence in varying the pneumonia-rate. Thus, in New York City from 1840 to 1858 (eighteen years) the mortality from pneumonia was 5.85 per cent., while from 1859 to 1877, inclusive, it was 6.2 per cent.
Before considering the exciting causes of croupous pneumonia, or their relation to its predisposing causes, the question meets us, Is croupous pneumonia an acute specific constitutional (infectious) disease or a local inflammation?12
12 Virchow's Archiv, Bd. lxx., Heidenhain.
That it is not a simple local inflammation appears from the following facts: the experiments with the inhalation of hot air, moist warm air, icy-cold air, vapors of various noxious acids and gases;13 the tracheal injection of caustic ammonia14 and mercury; and traumatism,—have all resulted negatively as exciting causes. And these experiments have all the more weight since they have been conducted not only at different times, and in countries distant from each other, but also because they have been repeated by various pathologists, and always with a similar result—viz. the development of lobular or catarrhal, and not of croupous, pneumonia. Section of the vagi certainly produces hepatization of the lungs, but it is not the hepatization of croupous pneumonia. Its distinctive microscopical characteristics are always wanting in the part of the lung consolidated by such experiments. A strong argument of those who adhere to the local theory of pneumonia is, that cold occupies a prominent place in its production. As exposure to cold and to draughts is a common experience, it is easy to ascribe the origin of any disease to cold.
13 Sityl, K. K. Akad. zu Wien, 867, Reitz.
14 Gendrin, Hist. Anat. des Inflam.
"Close rooms and bad air," says Squire, "more predispose than does outdoor exposure, unless that be prolonged or the individual resistance weakened by fatigue or intemperance." Both wet and cold invariably heighten the bronchitis-rate and exacerbate catarrhal processes, but neither of these can be proven to influence the pneumonia-rate. Statistics show that croupous pneumonia is more prevalent in our Southern States than in our Northern States. The epidemics in the West Indies are as well known as, and have been more devastating than, those in Iceland and in the Norse countries. The prevalence of pneumonia in this continent progressively increases from the pole to the equator.