The Collaborative International Dictionary of English v.0.48:

Unwellness \Un*well"ness\, n.
   Quality or state of being unwell.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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