If something is in a state of deterioration, it's getting worse. A crumbling old house with a caved-in roof and foot-tall weeds growing through the deck is going through deterioration.
Deterioration implies that things are falling apart: something once in good condition is now weakened, worn out, or otherwise in decline. Things often undergo deterioration because of neglect. Time also has a lot to do with deterioration. Museum curators are always worried about stopping the deterioration of their prized antiquities, like the ancient Egyptian mummies that should not be exposed to air.