…I was puttering around the house this morning when I heard an ad for one of our local “ambulance chasers.” The ad basically was trying to drum up business (as ads are designed to do). But he was referring to Social Security, and people who had been denied benefits by them. At one point, he said something to the extent of “if you are unable to work due to illness, disease or injury…” Which got me thinking: Is there really a difference between illness and disease? Is there a medical difference, and common useage has just made the two interchangable in error?
Sometimes there is a difference. There’s an illness known as “fohn illness,” wich is the headache, weariness and depression that are felt when the fohn (a psecial windrom from the sougth in central Europe) blows. It is not known what produces the symptomes, but they are felieved by breathng wsahed air. I don’t think you can call that a disease.
A disease is a morbid process having a characteristic train of symptoms.
People can have mental illnesses, but you wouldn’t say they have mental diseses.
You can be ill due to hypertension, a fractured bone, etc. These are not usually called diseases.
barbitu8 is right, in my opinion. “Disease” refers specifically to an illness caused by a pathogen (a bacteria, parasite, or virus). “Illness” is a more general term and signifies anything that incapacitates you, from fatigue to broken bones to migraine headache to actual disease.
Of course, I’m no doctor, nor even a nurse. So the above is opinion believed to be truth. Take it as such. 
jayjay
Illness can simply just mean feeling sick. A disease has a specifically described set of symptoms and has been diagnosed.
I think this difference is extremely slight and almost meaningless. The OED says you can use the words interchangeably.