The distinction between disease and illness was not so obvious to me. Growing up I always interchanged the two words and never really thought twice about it. Having a family that was not very educated in the medical field probably did not help. We just relayed whatever our doctors said. After the lectures and readings I now think of illness as anything that prevents you from living life in a healthy way (health being defined as a balance of all systems of the body). Illnesses can be mental or physical, and it comes from a disease. A disease is the actual cause of harm to the body. For instance, if you were bit by a poisonous spider, the poison in your blood would be the disease, and the disease would bring you fatigue and sweating (the illness).
With that being said, I would say that “the flu,” as spoken of through society, is an illness. When your friend says they have “the flu” you better believe they are feeling extremely yucky with cold sweats and fevers. Meanwhile, “Influenza” would be the disease causing these feelings.
I think that how the term “sadness” is used depends on whether it would be an illness or not. Of course one would be sad during a break up, or if their dog died, or if they lost some money. But it becomes an illness when they are sad all the time – when it is affecting their eat habits, sleep patterns, and overall lifestyle. In this sense, they might have depression or something of that sort, and sadness would be considered their illness.