The Collaborative International Dictionary of English v.0.48:

deteriorate \de*te"ri*o*rate\ (d[-e]*t[=e]"r[i^]*[-o]*r[=a]t),
   v. i.
   To grow worse; to be impaired in quality; to degenerate.
   [1913 Webster]

         Under such conditions, the mind rapidly deteriorates.
                                                  --Goldsmith.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Deteriorate \De*te"ri*o*rate\ (d[-e]*t[=e]"r[i^]*[-o]*r[=a]t),
   v. t. [imp. & p. p. Deteriorated
   (d[-e]*t[=e]"r[i^]*[-o]*r[=a]`t[e^]d); p. pr. & vb. n.
   Deteriorating (d[-e]*t[=e]"r[i^]*[-o]*r[=a]`t[i^]ng).] [L.
   deterioratus, p. p. of deteriorare to deteriorate, fr.
   deterior worse, prob. a comparative fr. de down, away.]
   To make worse; to make inferior in quality or value; to
   impair; as, to deteriorate the mind. --Whately.
   [1913 Webster]

         The art of war . . . was greatly deteriorated.
                                                  --Southey.
   [1913 Webster]

WordNet (r) 3.0 (2006):

deteriorate
    v 1: become worse or disintegrate; "His mind deteriorated"
    2: grow worse; "Her condition deteriorated"; "Conditions in the
       slums degenerated"; "The discussion devolved into a shouting
       match" [syn: devolve, deteriorate, drop, degenerate]
       [ant: convalesce, recover, recuperate]