1 : (no object) : to become worse as time passes
The weather gradually deteriorated (=worsened) as the day went on.
Her health continues to deteriorate. (=decline)
efforts to save a deteriorating rain forest
The disagreement deteriorated into a fight. (=the disagreement got worse and eventually became a fight)
2 : (+ object) : to make (something) worse
Exposure to rain and sun will gradually deteriorate the paint.
— deteriorated adjective
a deteriorated (=run-down, dilapidated) building
a badly deteriorated part of the city