hyperpyrexia

from WordNet (r) 3.0 (2006)

hyperpyrexia
    n 1: extremely high fever (especially in children)

from The Collaborative International Dictionary of English v.0.48

Hyperpyrexia \Hy`per*py*rex"i*a\, n. [NL. See {Hyper-}, and
   {Pyrexia}.] (Med.)
   A condition of excessive fever; an elevation of temperature
   in a disease, in excess of the limit usually observed in that
   disease.
   [1913 Webster]