Illness, n.
Etymology: [From Ill.]
1. The condition of being ill, evil, or bad; badness; unfavorableness. [Obs.] "The illness of the weather." Locke.
2. Disease; indisposition; malady; disorder of health; sickness; as, a short or a severe illness.
3. Wrong moral conduct; wickedness. Shak.
Syn. -- Malady; disease; indisposition; ailment. -- 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."
Shades of meaning
Fernald has no entry of its own for illness; he lists it under disease.
Where he writes about it:
There is now, in England, a tendency to restrict the words sick and sickness to nausea, or "sickness at the stomach," and to hold ill and illness as the only proper words to use in a general sense. — from disease
We speak of trifling ailments, a slight indisposition, a serious or a deadly disease; a slight or severe illness; a painful sickness. — from disease