It goes on to give some more metaphorical meanings, which is interesting BTW since "depravity" is among them. Anyhow . . .a disordered or incorrectly functioning organ, part, structure, or system of the body resulting from the effect of genetic or developmental errors, infection, poisons, nutritional deficiency or imbalance, toxicity, or unfavorable environmental factors; illness; sickness; ailment.
You may already have concluded that this definition isn't very, well, definitive. The list of causes isn't helpful since, between genetic or developmental errors and unfavorable environmental factors we have exactly everything that can possibly happen to us. And "disordered or incorrectly functioning" just begs the question. What is order or correct functioning? There is actually another problem with this definition. As weakly as it seems to rule in or out, it actually fails to include many diseases that we do recognize, because when we name something a disease, we can't necessarily point to a specific organ, part, structure or system which is disordered or incorrectly functioning in some way that we understand. So it's not just useless, it's wrong.
For what it's worth, here's what I think we mean when talk about disease, in a medical sense.
The first category is fairly clear cut. These are situations that fulfill the definition, in which we can pretty much agree intuitively that one of our parts or processes is not functioning correctly. We don't like it when we have constant pain, or can't do something that most people can do, or die otherwise than in our sleep at age 85. If we can confidently attribute the cause to a known physical property of our body and its functioning, we can name a disease and not get much of an argument. For example, if a bacterium is eating our lungs, we can name that, and hopefully if we take the right antibiotic we can end up being cured. We had pneumonia, or TB, now we don't. Easy.
But we start to have a problem with that comparator what "most people can do." Three problems actually. The first is the threshold of "most." How uncommon or far from the norm do you have to be before you merit a disease label? For example, what used to be called mental retardation, now more politely called cognitive disability or limitation, is defined completely arbitrarily, by a measured IQ of 70 or less. Give the same person an IQ test tomorrow, it might be 75. And what's the difference between 69 and 71? There's a difference alright: if it's 69, they can't execute you, but you can get special ed.
The second is what abilities really matter. I can't grow hair on the top of my head. Is that a disease?
The third is that we are all, every one of us, born with a hereditary, incurable, inevitably fatal condition that over time robs us of our physical and mental capacities. After age 45 or so, you will need reading glasses. Your ability to hear high frequencies will decline. Your joints will start to ache. You will lose lean muscle mass. I don't want to continue with all this depressing stuff, you can add to this list as you like, but the point is, where do you draw the line between having diseases and the human condition of mortality? In every one of these cases, we know quite well what physical processes are responsible, but are they "incorrect" or "disordered"? I'm not sure, but doctors certainly will treat all of these conditions.
Then we have situations in which we aren't presently experiencing any misery whatsoever but doctors say we have a disease because something about us puts us at risk of misery in the future: type 2 diabetes, hypertension, hypercholesterolemia, that sort of thing. Again, these are usually defined with some arbitrary threshold on a test of some sort.
And of course as I've discussed earlier we have diseases which are labels for clusters of symptoms, often largely consisting of self-reported experiences, for which no specifically disordered or incorrectly functioning organ, system or process is known. This is particularly characteristic of psychiatric "disorders" but there are some in other fields of medicine. For most of these we also have the earlier problems of locating the threshold of diagnosable abnormality and distinguishing the inevitable pain of existence from something that needs to be cut out of us by professional intervention. To the problem of psychiatric diagnosis we also have to add that some of them are labels for the way other people feel about the patient, and are not in fact distressing to the patient, e.g. narcissistic personality disorder.
So, before we can even do these clinical trials, we need to have definitions and labels for diseases, and for the amelioration thereof. But to what extent is that a scientific question, and to what extent a moral or cultural quandary? Doctor-think is pretty much exclusively done in disease categories. It may be helpful, even necessary, but it can also be limiting, and it can be used to sell us pills and other stuff we might be better off without.
PS: in case our friend Ana is reading, I'll be in Basel August 19-22 for the international environmental health conference. Let me know if you can do lunch.