Scarlatina, an infectious disease (7,5)
I believe the answer is:
scarlet fever
I believe this is a double definition.
'scarlatina' is the first definition.
(synonyms)
'an infectious disease' is the second definition.
(I know that scarlet fever is a Infectious disease)
(Other definitions for scarlet fever that I've seen before include "complaint" , "Infectious disease producing rash" , "Disease causing rash" , "Infectious disease of children" .)