The Collaborative International Dictionary of English v.0.48:

Relapsing \Re*laps"ing\, a.
   Marked by a relapse; falling back; tending to return to a
   former worse state.
   [1913 Webster]

   Relapsing fever (Med.), an acute, epidemic, contagious
      fever, which prevails also endemically in Ireland, Russia,
      and some other regions. It is marked by one or two
      remissions of the fever, by articular and muscular pains,
      and by the presence, during the paroxism of spiral
      bacterium (Spirochaete) in the blood. It is not usually
      fatal. Called also famine fever, and recurring fever.
      [1913 Webster]

WordNet (r) 3.0 (2006):

relapsing fever
    n 1: marked by recurring high fever and transmitted by the bite
         of infected lice or ticks; characterized by episodes of
         high fever and chills and headache and muscle pain and
         nausea that recur every week or ten days for several months
         [syn: relapsing fever, recurrent fever]