Disease that’s spread on the skin right away (8)

I believe the answer is:

epidemic

I'm a little stuck... Click here to teach me more about this clue!

'disease' is the definition.
(I've seen this before)

I can't explain the remainder of the clue.

(Other definitions for epidemic that I've seen before include "Citrus fruit" , "Unfortunately widespread" , "Disease affecting many people simultaneously" , "Plague" , "Dim piece (anag)" .)

I've seen this clue in The Guardian.

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