relapse

Short definition from Wiktionary

Verb

1. 1
   To fall back again; to slide or turn back into a former state or practice.
2. 2
   To recur; to worsen, be aggravated (after a period of improvement).
3. 3
   To return to a vice, especially self-harm or alcoholism, failing to maintain abstinence.
4. 4
   To slip or slide back physically; to turn back.

Noun

1. 1
   The act or situation of relapsing.
2. 2
   An occasion when a person becomes ill again after a period of improvement
3. 3
   One who has relapsed, or fallen back into error; a backslider.