Answer:

worsen

Answer	Crossword Clue
WORSEN	Deteriorate
WORSEN	Decline
WORSEN	Take a bad turn

WORSEN	Head south, so to speak
WORSEN	Go downhill
WORSEN	Slip
WORSEN	Deteriorate.
WORSEN	Start to go to pot
WORSEN	Go downhill, in a way
WORSEN	Exacerbate
WORSEN	Become more severe

WORSEN	Go up on the Saffir-Simpson scale