In pneumonic cases patients no doubt spread it around them by coughing, and others may take it up through the air-passages or the skin; but even then the range of infection is small, and such cases are comparatively rare.
At a very early period it was held by Virchow that the large cheesy masses found in tuberculosis of the lung are to be regarded as pneumonic infiltrations of the air-vesicles.
In pneumonic cases it is presumed to enter by the air-passages, and in bubonic cases by the skin.
In plague countries the diseases with which it is most liable to be confounded are malaria, relapsing fever and typhus, or broncho-pneumonia in pneumonic cases.
There is general engorgement and oedema of the lungs, with pneumonic patches varying in size and irregularly distributed.
As might be expected from these considerations, the bubonic type is very little infectious, while pneumonic cases are highly so, the patients no doubt charging the surrounding atmosphere by coughing.
When it is not prevalent the diagnosis is never easy, and in pneumonic and septicaemic cases it is impossible without bacteriological assistance.
Their pneumonic nature has been amply substantiated in later times; they are now regarded simply as evidence of pneumonic reaction to the stimulus of the tubercle bacillus.
The melancholy incident illustrates several points of interest: (1) the correctness of the bacterial theory of causation, and the identity of the bacillus pestis as the cause; (2) the infectious character of the pneumonic type of disease; (3) its high fatality; (4) the difficulty of diagnosis.