Plural of febres; occurrences of fever or feverish conditions (archaic or Latin plural).
From Latin 'febris' meaning 'fever,' used in medieval Latin medical texts; the plural 'febres' reflects Latin plural formation, though rarely used in modern English.
Medical and scientific English is full of Latin plurals like this—'febres' instead of 'fevers'—and doctors still use them, which is why you see phenomena/phenomenon and bacteria/bacterium in scientific contexts.
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