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]