Serious illness — Mark felt awful continually (7,5)

I believe the answer is:

scarlet fever

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'serious illness' is the definition.
'scarlet fever' can be an answer for 'illness' (scarlet fever is a kind of illness). I am not certain of the 'serious' bit.

'mark felt awful continually' is the wordplay.
'mark' becomes 'scar' (mark left by an injury).
'awful' indicates anagramming the letters.
'continually' becomes 'ever' (ever can mean always).
'felt' anagrammed gives 'letf'.
'scar'+'letf'+'ever'='SCARLET FEVER'

Can you help me to learn more?

(Other definitions for scarlet fever that I've seen before include "Infectious disease of children" , "Infectious disease producing rash" , "Disease causing rash" , "complaint" .)

I've seen this clue in The Times.

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