ill vs sick

no real difference.

As a short answer, "no real difference" is fine as a simple rule of thumb. However, there are some distinctions in usage. Examples:

--In the idiomatic expression "sick and tired of", the word "sick" would never be replaced with "ill".
Example: "I'm sick and tired of doing all the work around here."
But never: "I'm ill and tired of doing all the work around here."

--My experience is that "sick" is the more commonplace word in everyday settings. Example: "Brunhilde is out sick and can't come to work today, but she has plenty of sick leave left." (never "ill leave")

--In its noun form, "illness" is used in more formal contexts, such as "mental illness" or "the incidence of illness in that population group".

<shrugs>

OK