The Collaborative International Dictionary of English v.0.48:

Prophylaxis \Proph`y*lax"is\, n. [NL. See Prophylactic.]
   (Med.)
   The art of preserving from, or of preventing, disease; the
   observance of the rules necessary for the preservation of
   health; preservative or preventive treatment.
   [1913 Webster]

WordNet (r) 3.0 (2006):

prophylaxis
    n 1: the prevention of disease

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "prophylaxis":
   active treatment, adjuvant treatment, antigen treatment,
   causal treatment, conservative treatment, cross-firing,
   diathermic treatment, dietetic treatment, drip treatment,
   drug treatment, empiric treatment, epidemiology,
   expectant treatment, fever treatment, health physics,
   heat treatment, hot-air treatment, hygiene, hygienics,
   light treatment, mental hygiene, palliative treatment,
   perennial treatment, preseasonal treatment, preventive dentistry,
   preventive medicine, preventive treatment, prophylactic psychology,
   prophylactodontia, prophylaxy, public health,
   radiotherapeutic treatment, rational treatment, sanitation,
   shock treatment, specific treatment, starvation treatment,
   supporting treatment, surgical treatment, symptomatic treatment,
   tonic treatment, vibration treatment