Yours truly -- fool to catch the French disease (7)
I believe the answer is:
measles
'disease' is the definition.
(measles is a kind of health problem)
'yours truly fool to catch the french' is the wordplay.
'yours truly' becomes 'me' (the person who set this crossword).
'fool' becomes 'ass' (both can mean a stupid person).
'to catch' indicates putting letters inside.
'the french' becomes 'le' ('the' in French).
'ass' going around 'le' is 'asles'.
'me'+'asles'='MEASLES'
(Other definitions for measles that I've seen before include "Disease suffered in childhood" , "Viral disease once common in children" , "Spotty viral infection" , "Rash-causing disease" , "Viral disease with rash" .)