Disease expert treated visitor stuffed with Christmas cake? (10)

'disease expert' is the definition.
(expert in the spread of diseases)

'treated visitor stuffed with christmas cake?' is the wordplay.
'treated' indicates anagramming the letters.
'stuffed with' indicates putting letters inside (some letters get stuffed inside).
'christmas cake?' becomes 'log' (as in a Yule log).
'visitor' with letters rearranged gives 'viroist'.
'viroist' placed around 'log' is 'VIROLOGIST'.