Disease has everything to do with their sustainability. Our societies don't exist in a vaccume. It's great to make the arguement that the societies - if they were never influenced by any other outside societies in any way, shape or form - would be perfectly sustainable. Unfortunately, and I hate to burst your bubble, the world doesn't work that way.
The problem is that to be truely sustainable we would need the following:
- A closed system. This means that nothing outside of the system would be intruduced. No other societies, no germs, animals, plants, artifacts, ect. that aren't already present.
- Only the use of renewable resources, and only at a rate such that the renewable resources consumed, over the long term, equal the resources replenished.
- That's it really.
Unfortunately for the proponents of this system:
- The world, and by definition the subsections of the world, are an open system. Things can be added from the outside, via Meteor on the larger level, or via migration on the smaller scale.
- If something non-renewable is consumed and no longer reusable or renewable - even one picogram of iron, or one cubic microliter of water over 100 years... the society is not permanently sustainable.
- The sun (a part of the open system, but we'll ignore that for now) is not an infinitely renewable resource, and will burn out in 300 billion years or so. Thus, using up sunlight (photosynthesis) is not sustainable.
There is no way for any society to be sustainable even from an academic perspective, unless they manage to find an infinite amount of resources.
"Civilization" such as it is, gives us the best shot at aquiring more resources at the cost of speeding up the process of unsustainability.