What is a disease?
It's an interesting question.
The most obvious answers would be along the lines of something which reduces functionality, or which causes pain, being a disease - but these are not really tenable biologically.
Biology is not about performing a function, nor is it about remaining pain free.
For example, parasites don't really perform a 'function' - and there are an awful lot of parasites in nature. All of the viruses are cell parasites, for a start - and you can just work-up from them. In a sense, all animals are parasites on plants.
And much of biology is about deliberately risking, even self-inflicting, pain - for some over-arching purpose. For example, stags and bull elephants will fight and sometimes die to become the dominant male; spawning salmon will swim up river until they die of exhaustion.
Of course that thing which all these living creatures do - what all parasites do, what all animals that risk pain and death do - is reproduce.
Reproduction is the imperative, in the sense that it is only the entities that successfully reproduce which we are in a position to talk about.
So, the bottom-line definition of disease must be along the lines of 'that which impairs reproduction'.
When it comes to humans, it is the effect on impairing reproduction which most conveniently unites all solid examples of disease - and this applies to both to physical and to psychological diseases such as schizophrenia, psychotic depression, dementias.
Because (real) diseases which do not kill prematurely, or cause suicide, will (on average) reduce reproductive success either by premature death, or by some other means such as the effect on sexual selection - as when skin diseases make an individual unattractive/ repulsive and unable to attract a long-term mate.
All of this is a prelude to a consideration of the fact of sub-replacement fertility on average in every developed nation in the world; and grossly sub-replacement fertility in some groups (the wealthiest, the most intelligent, the least religious) - and especially among women of these groups.
Is this evidence of disease in such societies and such specific groups?
Yes of course it is.
Does it make any difference that the sub-fertility is chosen rather than a result of incapacity?
No it doesn't - the same applies to psychiatric diseases such as schizophrenia, melancholia and any illness which leads to suicide - these are instances of chosen low fertility.
Especially among women - since any fertile woman can conceive if she chooses to get pregnant, and in modern society almost all babies born will survive to an age when they themselves can reproduce.
On strict biological grounds, when chosen sub-fertility happens on average in a group over a generational timescale, it must be due to psychiatric disease.
So is modernity a disease?
Of course modernity is a disease, a psychiatric disease; and a very severe psychiatric disease at that - since it has the same effect as a severe psychiatric disease in generating very significant chosen sub-fertility.
Indeed the severity of modernity as a psychiatric disease is likely to be more extreme than in most psychiatric patients, since the prosperity and conditions of the average citizen in a developed nation are vastly better for child rearing than the average severe psychiatric patient - and yet reproduction is rejected.
So the conclusion seems inescapable that in conditions of modernity there is conclusive objective evidence of endemic, near universal, and very severe psychiatric disease.
Look around - that is what we see almost everywhere. We are living in a madhouse!
Note: I suppose I should add what I believe to be the nature of the severe, endemic psychiatric disease that afflicts modernity: it is addiction. Moderns are addicted to distractions - pleasurable distractions for preference, but lacking pleasures then any kind of distraction will suffice. Addicts orientate their lives around getting the drug they crave, ignoring normal human instincts, imperatives, aversions. We resemble the 'wireheaded' rats that press a lever to stimulate the pleasure centre in their brains - a compulsion that over-rides eating, drinking, sex, sleep... http://wireheading.com/james-olds.html