It is found in the buboes in ordinary cases, in the blood in the so-called " septicaemic " cases, and in the sputum of pneumonic cases.
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.
(3) In septicaemic cases the symptoms are those of the bubonic type, but more severe and without buboes.
When it is not prevalent the diagnosis is never easy, and in pneumonic and septicaemic cases it is impossible without bacteriological assistance.
(3) Septicaemic cases.
In 21 experiments out of 38, 55% of healthy rats living in flea-proof cages have contracted plague after receiving fleas collected from rats either dead or dying of septicaemic plague; consequently it is proved the rat flea can transmit plague from rat to rat.