The Collaborative International Dictionary of English v.0.48:

Sickness \Sick"ness\, n. [AS. se['o]cness.]
   1. The quality or state of being sick or diseased; illness;
      sisease or malady.
      [1913 Webster]

            I do lament the sickness of the king. --Shak.
      [1913 Webster]

            Trust not too much your now resistless charms;
            Those, age or sickness soon or late disarms. --Pope.
      [1913 Webster]

   2. Nausea; qualmishness; as, sickness of stomach.
      [1913 Webster]

   Syn: Illness; disease; malady. See Illness.
        [1913 Webster]

WordNet (r) 3.0 (2006):

sickness
    n 1: impairment of normal physiological function affecting part
         or all of an organism [syn: illness, unwellness,
         malady, sickness] [ant: health, wellness]
    2: defectiveness or unsoundness; "drugs have become a sickness
       they cannot cure"; "a great sickness of his judgment"
    3: the state that precedes vomiting [syn: nausea, sickness]

Moby Thesaurus II by Grady Ward, 1.0:

125 Moby Thesaurus words for "sickness":
   aberration, abnormality, acute disease, affection, affliction,
   ailment, alienation, allergic disease, allergy, atrophy,
   bacterial disease, birth defect, blight, brain damage,
   brainsickness, cardiovascular disease, chronic disease,
   circulatory disease, clouded mind, complaint, complication,
   condition, congenital defect, craziness, daftness, defect,
   deficiency disease, deformity, degenerative disease, dementedness,
   dementia, derangement, disability, disease, disorder,
   disorientation, distemper, distraction, endemic, endemic disease,
   endocrine disease, epidemic disease, folie, functional disease,
   fungus disease, furor, gastrointestinal disease, genetic disease,
   handicap, hereditary disease, iatrogenic disease, ill, ill health,
   illness, indisposedness, indisposition, infectious disease,
   infirmity, insaneness, insanity, irrationality, loss of mind,
   loss of reason, lunacy, madness, malady, malaise, mania,
   mental deficiency, mental derangement, mental disease,
   mental disorder, mental disturbance, mental illness,
   mental instability, mental sickness, mind overthrown, mindsickness,
   morbidity, morbus, muscular disease, neurological disease,
   nutritional disease, occupational disease, oddness,
   organic disease, pandemic disease, pathological condition,
   pathology, pixilation, plant disease, possession,
   protozoan disease, psychosomatic disease, queerness, rabidness,
   reasonlessness, respiratory disease, rockiness, secondary disease,
   seediness, senselessness, shattered mind, sick mind, sickishness,
   signs, strangeness, symptomatology, symptomology, symptoms,
   syndrome, the pip, unbalance, unbalanced mind, unhealthfulness,
   unhealthiness, unsaneness, unsound mind, unsoundness,
   unsoundness of mind, urogenital disease, virus disease,
   wasting disease, witlessness, worm disease