My definition would be any individual being in the normative state and free from any kind of disease or illness. I know its kind of a broad definition but it works pretty well too, it means that you are not suffering any kind of pathogenic disease and that your body is functioning properly and with out any issues. It also means that your mental state is normal too. Normal can have different meanings and definitions for different people but in this case it would mean was is generally considered normal for your society and culture.
Illness on the other hand I think is pretty much the opposite. Illness is the state of being anywhere away from “normal” whether that is biological, psychological, or even socially and culturally.
I really feel like these ideas came from how I was raised and is just part of the American culture. We generally see health as being able to function completely and illness as somehow being wrong or off and in some way not being able to function like everyone else.
The Flu: I consider this a definite disease, not an illness. it is something that is caused by the pathogenic inflenza virus that causes the bodies response to it. Definitely a disease.
Cancer: Again, I would consider this one a disease and not an illness. Cancer is caused by a genetic mutation and some problem in the chromosome. This mutation grows and effects many systems of the body. While there are definitely some psychological and societal attributes of cancer, it is definitely more of a disease.
Anxiety: this is one that I would consider more of an illness. Though it has some physical attributes I would say that it is largely a psychological and social effector. I would say that because it effects more in these realms that it is an illness and not a disease.