WordNet sense 1 (somewhat ill or prone to illness):

ailing, indisposed, peaked, poorly, seedy, unwell, peaked, under the weather

WordNet sense 2 (affected by an impairment of normal physical or mental function):

ill, sick

WordNet sense 3 (unhealthy looking):

sallow

WordNet sense 4 (not in or exhibiting good health in body or mind):

unhealthy