My mother’s illness still hasn’t improved.
I got a disease that makes me fart all the time, so I've haven't been able to go outside.
"Can you fix my mom’s illness, doctor?" "I’ll give it a try, but don’t count on it."
When your energy is that of sickness, you are probably sick.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.