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All answers having "fever" as clue:

3 Character Answers:

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HAY



4 Character Answers:

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AGUE

HEAT

RANA



5 Character Answers:

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CABIN

CRAZE

MANIA



6 Character Answers:

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TYPHUS



7 Character Answers:

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HOTNESS

MALARIA



15 Character Answers:

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HIGHTEMPERATURE