Seasonal bug (3)
I believe the answer is:
flu
'seasonal bug' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for flu that I've seen before include "Viral infection (colloq.)" , "Minor ailment" , "Winter ailment" , "Highly contagious viral disease" , "Swine --; gastric --" .)