Sickness from epidemic 18 got rid of (4)
I believe the answer is:
ague
'sickness' is the definition.
(ague is a kind of sickness)
I cannot understand how the rest of the clue works.
(Other definitions for ague that I've seen before include "Malarial fever; chill" , "Feverish illness" , "Fit of shivering and shaking" , "Burning fever" , "Acute fever; shivering fit" .)