"ill" came from a Germanic language, Norse, and means a bad condition or thing; ill will, ill fitting, for example. "disease" is from Old French and dis means down or away from, and ease I think you know. It's dis + ease; away from easy which is what happens to organs or parts of the body. English is a mix of several other languages; there is a history of words in the book "keys to words" at: http://www.bdmadden.com/ Good Q!

An illness is more vague. It is generally a condition that causes pain and discomfort. Disease refers to the specific cause of the illness. Also, an illness is usually curable. The term "disease" can be more serious, referring to curable sicknesses, but also to those that can cause death.

Illness, is mostly sickness like the flu. Diseases are usually contagious.