Need help deciphering the meaning behind a phrase spoken in 1893

meaning-in-context etymology meaning vocabulary linguistics

As best as I can tell, in this context, humour means 'mood or state of mind' (Lexico). So Prof. von Pettenkofer is using a very convoluted way, with unnecessarily 'big' words, to say that the study of bacteria is 'in', is fashionable, is something many people are obsessed by. The wittiness of the phrase comes from the convoluted phrasing.

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