The Collaborative International Dictionary of English v.0.48:

Prevalence \Prev"a*lence\, n. [L. praevalentia: cf. F.
   pr['e]valence. See Prevail.]
   The quality or condition of being prevalent; superior
   strength, force, or influence; general existence, reception,
   or practice; wide extension; as, the prevalence of virtue, of
   a fashion, or of a disease; the prevalence of a rumor.
   [1913 Webster]

         The duke better knew what kind of argument were of
         prevalence with him.                     --Clarendon.
   [1913 Webster]

WordNet (r) 3.0 (2006):

prevalence
    n 1: the quality of prevailing generally; being widespread; "he
         was surprised by the prevalence of optimism about the
         future"
    2: (epidemiology) the ratio (for a given time period) of the
       number of occurrences of a disease or event to the number of
       units at risk in the population
    3: a superiority in numbers or amount; "a preponderance of
       evidence against the defendant" [syn: preponderance,
       prevalence]

Moby Thesaurus II by Grady Ward, 1.0:

117 Moby Thesaurus words for "prevalence":
   abundance, acceptance, accustomedness, affluence,
   ample sufficiency, ampleness, amplitude, ascendancy, avalanche,
   averageness, bon ton, bonanza, bountifulness, bountiousness,
   bumper crop, common occurrence, commonality, commonness,
   commonplaceness, confirmedness, control, copiousness, currency,
   customariness, extensiveness, extravagance, exuberance,
   fashionableness, fertility, fixedness, flood, flow, foison,
   frequence, frequency, full measure, fullness, generality,
   generosity, generousness, great abundance, great plenty, gush,
   habitualness, incidence, inveteracy, inveterateness, landslide,
   lavishness, liberality, liberalness, lots, luxuriance, mastery,
   maximum, modishness, more than enough, much, myriad, myriads,
   normality, numerousness, oftenness, omnipresence, opulence,
   opulency, ordinariness, outpouring, overflow, pervasiveness,
   plenitude, plenteousness, plentifulness, plenty, popularity,
   practice, predominance, primacy, prodigality, productiveness,
   profuseness, profusion, quantities, rampantness, reign,
   relative incidence, repleteness, repletion, rich harvest,
   rich vein, richness, rifeness, riot, riotousness, routineness,
   rule, run, scads, settledness, shower, spate, standardness, stream,
   stylishness, substantiality, substantialness, superabundance, sway,
   sweepingness, teemingness, ubiquity, universality, usualness,
   voguishness, wealth, widespreadness, wontedness