A person who is ill is sick. The words "ill" and "sick" have almost the same meaning.
He feels ill.
If you’re ill, you should just stay home and rest.
An ill patient requires a lot of attention.
I’m feeling kind of ill this morning.
The word "ill" has other slightly different meanings:.
Harold bears no ill will towards his ex-wife. (He doesn’t hate her or wish bad things happen to her.)
Criminals buried their ill-gotten gains outside in the backyard. (ill-gotten = illegally acquired)
Knowing that the bystanders could have saved that young woman’s life, and they did nothing, makes me feel ill. (makes me feel ill = it disturbs me)
She’s a woman of ill repute. (She has a bad reputation)
Ill-informed voters helped elect a man who wasn’t qualified for the job. (ill-informed = people who receive bad information or who are not well educated)
The word "illness" is a noun:
He has a severe illness.
Mary is suffering from a mysterious illness.
Mental illness is a problem that affects millions of people in the U.S.
Illness from the ebola virus is so serious that it can result in death.
