What is difference between Ill and Sick, How do I say "sick people" or "ill people" to refer to people not feeling well?
Honestly, I was not aware of the differences between both either; however, after searching for it for a while, I found out some interesting differences between both:
Is normally used to refer to long-term diseases, such as cancer, pneumonia and etc..
Can never be used attributively (before the noun) - ex: ill child. Instead, it can only be used postpositively (after the noun) or predicatively (after a linking or copula verb) ex: A child ill - He seems ill.
Ill also works as a prefix when implying something made in a bad manner: The ill-painting of my house, we should have never employed that guy!
Is an informal and short term, usually refers to short sickness such as one that unables you to go to school for one or two days.
Can be used attributively, postpositively and predicatively, ex: I saw a sick child; the child sick; the child seems sick.
Sick and ill are both adjectives that mean ‘not in good health’. We use both sick and ill after a verb such as be, become, feel, look or seem:
I was ill for a time last year, but I’m fine now.
Nancy looks ill. I wonder what’s wrong with her.
I felt sick and had to go home at lunchtime.
We can use sick before a noun but we don’t normally use ill before a noun:
She’s been looking after a sick child this week, so she’s not at work.
Not: … an ill child …
In American English sick means more generally ‘to be unwell’.