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]