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.
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.
Both died of pneumonic plague, from which also Barisch had undoubtedly suffered.
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.
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.
One of the results of recent observation is the classification of plague cases under three heads, which have already been mentioned several times: (1) bubonic, (2) pneumonic, (3) septicaemic. (The word " pesti-caemic " is also used instead of " septi-caemic," and though etymologically objectionable, it is otherwise better, as " septicaemic " already has a specific and quite different meaning.) It should be understood that this classification is a clinical one, and that the second and third varieties are just as much plague as the first.
(2) Pneumonic plague was observed and described in many of the old epidemics, and particularly by two medical men, Dr Gilder and Dr Whyte, in the outbreak in Kathiawar in 1816; but its precise significance was first recognized by Childe in Bombay.
When it is not prevalent the diagnosis is never easy, and in pneumonic and septicaemic cases it is impossible without bacteriological assistance.
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.
(2) Pneumonic cases.
There is general engorgement and oedema of the lungs, with pneumonic patches varying in size and irregularly distributed.
In pneumonic cases it is presumed to enter by the air-passages, and in bubonic cases by the skin.
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.
On this report it may, therefore, be taken that aerial infection, except, perhaps, in pneumonic cases, may be excluded, and that the chief source of infection is the flea.