One's favourite island has vitality that's contagious (8)
I believe the answer is:
impetigo
'that's contagious' is the definition.
I can't tell whether this defines the answer.
'one's favourite island has vitality' is the wordplay.
'one's' becomes 'I'm' ('one is' is a formal way of saying 'I am').
'favourite' becomes 'pet' (synonyms).
'island' becomes 'i' (geographical abbreviation).
'has' is a charade indicator (letters next to each other).
'vitality' becomes 'go' (I've seen this in other clues).
'im'+'pet'+'i'+'go'='IMPETIGO'
Can you help me to learn more?
(Other definitions for impetigo that I've seen before include "unsightly condition" , "sickness" , "Contagious skin disease with spots forming yellow crusts" , "Skin condition" , "Contagious skin infection common in children" .)