The Collaborative International Dictionary of English v.0.48:

Fester \Fes"ter\, v. i. [imp. & p. p. Festered; p. pr. & vb.
   n. Festering.] [OE. festern, fr. fester, n.; or fr. OF.
   festrir, fr. festre, n. See Fester, n.]
   1. To generate pus; to become imflamed and suppurate; as, a
      sore or a wound festers.
      [1913 Webster]

            Wounds immedicable
            Rankle, and fester, and gangrene.     --Milton.
      [1913 Webster]

            Unkindness may give a wound that shall bleed and
            smart, but it is treachery that makes it fester.
                                                  --South.
      [1913 Webster]

            Hatred . . . festered in the hearts of the children
            of the soil.                          --Macaulay.
      [1913 Webster]

   2. To be inflamed; to grow virulent, or malignant; to grow in
      intensity; to rankle.
      [1913 Webster]

WordNet (r) 3.0 (2006):

festering
    n 1: (medicine) the formation of morbific matter in an abscess
         or a vesicle and the discharge of pus [syn: festering,
         suppuration, maturation]
    2: a fluid product of inflammation [syn: pus, purulence,
       suppuration, ichor, sanies, festering]

Moby Thesaurus II by Grady Ward, 1.0:

128 Moby Thesaurus words for "festering":
   abscess, adulteration, algesia, algetic, angry, aposteme, bad,
   bed sore, befouling, blain, bleb, blister, boil, bubo, bulla,
   bunion, burning, canker, canker sore, cankered, carbuncle, carious,
   chafed, chancre, chancroid, chilblain, cold sore, contamination,
   corrupt, corruption, decayed, decomposed, defilement, envenoming,
   eschar, felon, fester, fever blister, fiery, fistula, foul,
   fouling, furuncle, furunculus, galled, gangrened, gangrenous,
   gathering, gleet, gone bad, gumboil, hemorrhoids, ichor, infection,
   inflamed, inflammation, irritated, irritation, kibe, lesion,
   leukorrhea, matter, mattering, misuse, mortified, necrosed,
   necrotic, papula, papule, paronychia, parulis, peccant,
   peccant humor, perversion, petechia, piles, pimple, pock,
   poisoning, pollution, polyp, prostitution, purulence, purulent,
   pus, pussy, pustule, putrefied, putrescent, putrid, rankling, raw,
   red, rising, rotten, rotting, running, sanies, scab, sensitive,
   sensitiveness, smarting, soft chancre, sore, sore spot, soreness,
   sphacelated, spoiled, stigma, sty, suppurating, suppuration,
   suppurative, swelling, tender, tenderness, tingling, tubercle,
   ulcer, ulcerated, ulceration, vitiation, wale, welt, wheal, whelk,
   whitlow, wound