“What is a disease?”
It would be nice to think that this is the kind of question where there are clear-cut, fact-based answers to be had. “Disease” is a term that seems to pick out a category of biological conditions, and biologists are pretty good with categorization.
A disease might be a particular physiological state that is incompatible with the proper functioning of an organism (say, because that state interferes with extracting nutrients from food, or expelling waste products, or oxygenating blood and moving it around the body). Or, from the geneticist’s point of view, a disease might be a bundle of traits (connected to a bundle of genes) that interferes with an organism’s ability in a particular environment to successfully reproduce before shuffling off this mortal coil. (Note that this makes sickle cell anemia – which is thought to provide some protection against malaria – a condition that counts as a disease in malaria-free environments but doesn’t count as a disease where malaria is endemic.)
Biological definitions of “health” as the proper functioning of a human body, or in terms of traits which contribute to reproductive success, would seem to be straightforward.
But humans are very peculiar organisms.
First off, we typically view our health in terms other than whether we’ve passed our genes on to the next generation. Indeed, many women control their fertility in the interests of their health, rather than having as many babies as possible.
More to the point, we have our own opinions about what it means for our bodies to be functioning properly or improperly. Some of this is influenced by our contact with health care providers, who may be able to detect issues like high blood pressure that have escaped our notice. Some of it is shaped by pharmaceutical advertising that makes us aware of newly classified syndromes for which there are now treatments (that just happen still to be on patent).