Illness rhymes with:
stillness, awfulness, coolness, dullness, fickleness, forcefulness, foulness, fullness, gentleness, harmfulness, idleness, peacefulness, playfulness, reasonableness, resourcefulness, smallness, stillness, truthfulness, usefulness, wastefulness, wellness, wistfulness, wonderfulness
Illness sounds like:
ilana's, illinois, illinois', illinois's, illnesses