No, not a noun. An adjective. As in "I am sick." And not ironic, either.
Just plain sick with temperature, cough, scratchy/sore throat, runny nose, sneezing, watery eyes. The works. I can't remember the last time I climbed into bed and let myself experience... SICKNESS. Noun, that.
I gave my class the day off. I can't see myself driving a car at 6:30 a.m. and/or getting "enthused" when I can't seem to string a set of words together without coughing into a fit.
Oy! Hope this leaves as fast as it arrived.