ill or sick?

In the case of the flu (influenza), SICK and ILL are both appropriate. I personally would use either one. If you want a recommendation I would use ILL.

That was the short answer, but I hate giving short answers, even to simple questions, so here's some more detail on ILL vs. SICK.

SICK and ILL are used in various standard combinations and phrases, where they are not interchangeab

When you have "the" flu .... Americans don't say they are ill, though they understand it perfectly. We are sick with the flu or have the flu. When we have the flu, we are sick. We get sick, meaning we acquire an illness. We understand what it means to fall ill, but it sounds literary to us.

Aha. ILL does sound a bit old-fashioned or British to me too. Even here, it's probably used more by older people.

thank u so much... that was helpful...:*

I don't understand the expression: "to use me ill" in the following sentence:
The books - the generous friends who met me without suspicion - the merciful masters who never used me ill!

Thank you

I haven't heard use me ill before, but it's clearly intended to mean abuse me, i.e. to use me in an inappropriate (ill) way.