Hirsch's statistics and statement, that "the amount of the mean fluctuation in the mortality from pneumonia is in inverse ratio to the density of the population," is a strong argument in favor of the view that croupous pneumonia is due to some specific pneumonic infection, for all acute general diseases increase where there is over-crowding.
It is often stated that pneumonia is a far more frequent disease now than it was twenty years ago. That I might arrive at something definite on this point, I have carefully examined the death-reports of England from 1840, also those of New York City, dividing them into two periods of eighteen years each; and I find that the average mortality from pneumonia in England from 1840 to 1858 was 5.57 per cent.; from 1859 to 1877, 4.77 per cent., an actual decrease of 14.3 per cent. In New York City from 1840 to 1858 the average ratio of mortality from pneumonia to all other diseases was 5.85 per cent., and from 1859 to 1877 it was 6.20 per cent., showing an increase in New York of 15.2 per cent. Thus it is shown that while in England pneumonia is on the decrease, in New York City it is on the increase.
Those who advocate cold as a cause of pneumonia base their argument on the seasons of the year when it is most prevalent. The winter and spring are pre-eminently the seasons of pneumonia, but cerebro-spinal meningitis, diphtheria, influenza, measles, and other diseases of similar general character occur with greatest frequency in the winter months, yet it is not now claimed that cold causes them. While it is not to be denied that cold is to a limited extent an exciting cause of pneumonia, the belief that it is the primary or principal cause must be held in abeyance if not altogether rejected. Again, the symptomatology of pneumonia seems to militate against its being a local disorder.
There is no relationship between the amount of lung-tissue involved and the intensity of the symptoms; high fever, delirium or convulsions, and rapid heart-failure are often as well marked when a post-mortem reveals only one lobe to be involved as when a double pneumonia exists. "The local inflammation in its gradual extension and composite character offers no sort of parallelism to the fever which for a while accompanies it." In local phlegmasiæ there is a direct ratio between the amount of surface involved and the attendant constitutional disturbances.
Rarely does a second chill occur when there is an extension of the pneumonic process. "Small consolidations with high fever and severe constitutional symptoms, and extensive infiltrations with a comparatively slight fever and general disturbance, are the rule and not the exception."15
15 Ziemssen's Cyclop. Prac. Med., vol. v. p. 146.
The absence of regular and constant prodromata, the absence of a known period of incubation, of a typical temperature-range, and of characteristic surface phenomena, and the fact that it is not contagious,—these must not be overlooked when we are tempted to regard croupous pneumonia as an acute infectious disease.
The points of resemblance between croupous pneumonia and the acute general diseases are the following: It has an initiatory chill, an orderly pyrexia, and a somewhat typical course, inasmuch as there are in many cases a day of abrupt crisis and a definite duration. The symptoms follow in regular sequence.
There is a peculiar countenance, and here we note a resemblance to typhus and typhoid; there are usually herpetic eruptions; the kidneys are not infrequently the seat of a nephritis; and catarrhal pyelitis is a common condition. The cerebral symptoms greatly resemble the condition that accompanies the exanthems. The peculiarity of its commencement in the very young and old—convulsions in the former and coma and collapse in the latter—serves to point to an alliance with those diseases where a specific morbific agent acts primarily and principally on the nervous system. Etiologically, it often arises under precisely similar circumstances as those which give origin to cerebro-spinal meningitis and diphtheria, to both of which diseases it is also allied, since the pathological changes are distinct from those of any other inflammation.
Again, the influence of septic, miasmatic, and atmospheric conditions is certainly almost universally acknowledged. A good example of this is the sewer-gas pneumonia so often occurring in New York City, and of which frequent mention is made by English writers. Again, there have been frequent epidemics of pneumonia in certain districts in garrisons and on board ship, where over-crowding, bad ventilation, and general antihygienic surroundings prevailed.16