Crossword Genius

Fever starts to fluctuate (3)

Ross

I believe the answer is:

flu

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'fever' is the definition.
The definition and answer can be both states of affairs as well as being singular nouns.
Perhaps there's an association between them I don't understand?

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for flu that I've seen before include "In brief, winter illness" , "Swine --; gastric --" , "Common ailment" , "A bad winter bug" , "Common annual illness" .)

I've seen this clue in The Globe and Mail.
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