The Collaborative International Dictionary of English v.0.48:

Infirmity \In*firm"i*ty\, n.; pl. Infirmities. [L. infirmitas
   : cf. F. infirmite. See Infirm, a.]
   1. The state of being infirm; feebleness; an imperfection or
      weakness; esp., an unsound, unhealthy, or debilitated
      state; a disease; a malady; as, infirmity of body or mind.
      [1913 Webster]

            'T is the infirmity of his age.       --Shak.
      [1913 Webster]

   2. A personal frailty or failing; foible; eccentricity; a
      weakness or defect.
      [1913 Webster]

            Will you be cured of your infirmity ? --Shak.
      [1913 Webster]

            A friend should bear his friend's infirmities.
                                                  --Shak.
      [1913 Webster]

            The house has also its infirmities.   --Evelyn.

   Syn: Debility; imbecility; weakness; feebleness; failing;
        foible; defect; disease; malady. See Debility.
        [1913 Webster]

WordNet (r) 3.0 (2006):

infirmity
    n 1: the state of being weak in health or body (especially from
         old age) [syn: infirmity, frailty, debility,
         feebleness, frailness, valetudinarianism]

Moby Thesaurus II by Grady Ward, 1.0:

220 Moby Thesaurus words for "infirmity":
   abnormality, abulia, acute disease, advanced age, advanced years,
   affection, affliction, age of retirement, ailment,
   allergic disease, allergy, an incurable disease, anility, atrophy,
   bacterial disease, bad habit, besetting sin, birth defect, blemish,
   blight, bug, cachexia, cachexy, caducity, cardiovascular disease,
   catch, chronic disease, chronic ill health, circulatory disease,
   complaint, complication, condition, congenital defect, cowardice,
   crack, danger, debilitation, debility, decay, decline of life,
   declining years, decrepitude, defect, defection, deficiency,
   deficiency disease, deformity, degenerative disease, delicacy,
   delicate health, desultoriness, disability, disease, disorder,
   distemper, dotage, drawback, eld, elderliness, endemic,
   endemic disease, endocrine disease, enervation, enfeeblement,
   epidemic disease, exhaustion, failing, failure, faintheartedness,
   faintness, fault, faute, fear, feeblemindedness, feebleness, flaw,
   foible, fragility, frailness, frailty, functional disease,
   fungus disease, gastrointestinal disease, genetic disease,
   green old age, hale old age, handicap, hazard, healthlessness,
   hereditary disease, hoary age, hole, hypochondria, hypochondriasis,
   iatrogenic disease, ill, ill health, illness, imperfection,
   inadequacy, incapacity, indisposition, infectious disease,
   infirm old age, infirmity of age, insecurity, insipidity,
   insolidity, instability, insubstantiality, invalidism, invalidity,
   kink, languishing, languishment, little problem, longevity, malady,
   malaise, moral flaw, morbidity, morbidness, morbus,
   muscular disease, neurological disease, nutritional disease,
   occupational disease, old age, oldness, organic disease,
   pandemic disease, pathological condition, pathology, peakedness,
   pensionable age, peril, perilousness, plant disease, pliability,
   poor health, precariousness, problem, protozoan disease,
   psychosomatic disease, respiratory disease, ricketiness, rift,
   ripe old age, risk, riskiness, rockiness, second childhood,
   secondary disease, seediness, senectitude, senility,
   senior citizenship, shakiness, shiftiness, shiftingness,
   shortcoming, sickishness, sickliness, sickness, signs,
   slipperiness, snag, softness, something missing, speculativeness,
   spinelessness, superannuation, symptomatology, symptomology,
   symptoms, syndrome, taint, the downward slope, the golden years,
   the pip, ticklishness, treacherousness, treachery, unauthenticity,
   unauthoritativeness, undependability, unfaithworthiness,
   unfirmness, unhealthiness, unreliability, unsolidity, unsoundness,
   unsteadfastness, unsteadiness, unsturdiness, unsubstantiality,
   unsureness, untrustworthiness, unwholesomeness, urogenital disease,
   vale of years, valetudinarianism, vapidity, vice, virus disease,
   vulnerable place, wasting, wasting disease, wateriness, weak link,
   weak point, weak side, weak will, weak-mindedness, weakening,
   weakliness, weakness, white hairs, wishy-washiness, worm disease