The Collaborative International Dictionary of English v.0.48:

Illness \Ill"ness\, n. [From Ill.]
   1. The condition of being ill, evil, or bad; badness;
      unfavorableness. [Obs.] "The illness of the weather."
      --Locke.
      [1913 Webster]

   2. Disease; indisposition; malady; disorder of health;
      sickness; as, a short or a severe illness.
      [1913 Webster]

   3. Wrong moral conduct; wickedness. --Shak.

   Syn: Malady; disease; indisposition; ailment.

   Usage: Illness, Sickness. Within the present century,
          there has been a tendency in England to use illness in
          the sense of a continuous disease, disorder of health,
          or sickness, and to confine sickness more especially
          to a sense of nausea, or "sickness of the stomach."
          [1913 Webster]

WordNet (r) 3.0 (2006):

illness
    n 1: impairment of normal physiological function affecting part
         or all of an organism [syn: illness, unwellness,
         malady, sickness] [ant: health, wellness]

Moby Thesaurus II by Grady Ward, 1.0:

73 Moby Thesaurus words for "illness":
   abnormality, acute disease, addiction, affection, affliction,
   ailment, allergic disease, allergy, atrophy, bacterial disease,
   birth defect, blight, bug, cardiovascular disease, chronic disease,
   circulatory disease, complaint, complication, condition,
   congenital defect, defect, deficiency disease, deformity,
   degenerative disease, disability, disease, disorder, distemper,
   endemic, endemic disease, endocrine disease, epidemic disease,
   functional disease, fungus disease, gastrointestinal disease,
   genetic disease, handicap, hereditary disease, iatrogenic disease,
   indisposition, infectious disease, infirmity, malady, malaise,
   morbidity, morbus, muscular disease, neurological disease,
   nutritional disease, occupational disease, organic disease,
   pandemic disease, pathological condition, pathology, plant disease,
   protozoan disease, psychosomatic disease, respiratory disease,
   rockiness, secondary disease, seediness, sickishness, sickness,
   signs, symptomatology, symptomology, symptoms, syndrome, the pip,
   urogenital disease, virus disease, wasting disease, worm disease