relapse

from The American Heritage® Dictionary of the English Language, 5th Edition.

• intransitive verb To return to a former state.
• intransitive verb To become sicker after partial recovery from an illness.
• intransitive verb To recur. Used of an illness.
• intransitive verb To slip back into bad ways; backslide.
• noun A return to a former state, especially after apparent improvement.

from The Century Dictionary.

• To slip or slide back; return.
• To fall back; return to a former bad state or practice; backslide: as, to relapse into vice or error after amendment.
• To fall back from recovery or a convalescent state.
• noun A sliding or falling back, particularly into a former evil state.
• noun One who has refallen into vice or error; specifically, one who returns into error after having recanted it.
• noun In medicine, the return of a disease or symptom during or directly after convalescence. See recrudescence.

from the GNU version of the Collaborative International Dictionary of English.

• noun A sliding or falling back, especially into a former bad state, either of body or morals; backsliding; the state of having fallen back.
• noun obsolete One who has relapsed, or fallen back, into error; a backslider; specifically, one who, after recanting error, returns to it again.
• intransitive verb obsolete To slip or slide back, in a literal sense; to turn back.
• intransitive verb To slide or turn back into a former state or practice; to fall back from some condition attained; -- generally in a bad sense, as from a state of convalescence or amended condition; ; -- sometimes in a good sense.
• intransitive verb (Theol.) To fall from Christian faith into paganism, heresy, or unbelief; to backslide.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• verb intransitive To fall back again.
• verb intransitive, medicine, of a disease To recur; to worsen, be aggravated.
• noun The act or situation of relapsing.
• noun obsolete One who has relapsed, or fallen back into error; a backslider.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• verb deteriorate in health
• noun a failure to maintain a higher state
• verb go back to bad behavior