UNICEF is to change and it's catching (10)
I believe the answer is:
Here is my best explanation:
'it's catching' is the definition.
('infectious' can be similar in meaning to 'catching')
'unicef is to change' is the wordplay.
'change' indicates anagramming the letters.
'unicefisto' with letters rearranged gives 'INFECTIOUS'.
'and' acts as a link.
(Other definitions for infectious that I've seen before include "Catching", "Liable to spread disease", "easily picked up", "(Of disease) liable to spread", "Easily transmitted" and "(Of disease) catchable from others".)