Invalid, victim of infectious disease initially (4)
I believe the answer is:
void
'invalid' is the definition.
(I've seen this before)
'victim of infectious disease initially' is the wordplay.
'initially' suggests taking the first letters.
The initial letters of 'victim of infectious disease' is 'VOID'.
(Other definitions for void that I've seen before include "Not valid or legally binding" , "Containing nothing or not legally binding" , "Vacant; not valid" , "Emptiness, empty space" , "Free" .)