I might just be misunderstanding the setting, but in a world where it is supposed to be literal fact that a lot of the disease and misfortune of medieval life is caused by a bunch of low-Might demons or other spirits running around causing mischief (pushing people off roofs, tempting people to community-harming sins, messing up their humors to make them sick, killing crops or rotting food stores and whatnot), shouldn't the Aegis of the Hearth inherently give covenfolk within it a somewhat higher standard of living? I'm not sure if that's just handwaved off because it would make an already-staple spell even stronger, or if it's a result of different writers having different ideas of the mechanics underpinning Mythic Europe, or perhaps the default assumption is that for as many little demons as there are running around causing problems there's an equal number of little angels running around helping people, so blocking out both is basically a wash?
I think you are right on that assumption that most of dieseases are caused by little/lesser deamons. But as deamons work they can posses a human and hide inside, and that way they can bypass the Aegis. After one year of hiding the deamon IS inside the new Aegis during casting - so not hindered by the new Aegis. After that the deamon is free to cause all kind of trouble.
The people of the covenant have to go outside of the covenant from time to time to get thing for the covenant. During those trips the deamons can posses them unfortunately.
The most important reason is that the living condition modifier is a core rules concept, and the core book does not have room to go into the details of disease demons and the risk of stepping out of the Aegis. Supplements add disease demons, but do not want to contradict core if it can be avoided.
And they can get away with this in any number of ways.
- Typical towns have -2 living conditions. Maybe the demons prefer towns.
- Maybe everybody steps outside the Aegis often enough that the demons have enough chances anyway.
- I love your idea that the Aegis also blocks the minor divine spirits.
It does not really matter why. There are enough reasons not to make the rules heavier than they already are.
You are right that logically the Aegis ought to block out disease demons and the like, and thereby ought to improve living conditions.
I think the actual reason the rules for the Aegis say nothing about this is a combination of "didn't think about that" and that the Aegis is largely inherited from early editions where the rules didn't contain such things as living conditions.
It is worth reading A&A p.45 box Demons of Disease and RoP:I p.32f Possession to compare then with the working of the AotH.
Possessed beings entering an AotH do not carry creatures with Might score. Neither is a possession by itself an effect to be resisted by an AotH.
So, while an AotH may still counteract effects caused by a possessing disease demon in the covenant, it does not banish the demon proper from the possessed being. As an AotH covers in general smaller areas, possessed people will enter and leave it without the AotH having much influences on their diseases.
"(...) a lot of the disease and misfortune of medieval life is caused by a bunch of low-Might demons (...)"
I think "a lot" is up for debate. Both from a reading of ArM p.180 and A&A p.45, I wouldn't say demons are necessarily a significant cause of disease. The percentage of diseases actually caused by demons seems to be up to the SG, and I think that in a game where you want to strongly emphasize the influence of demons on the lives of comon folks it makes sense for the Aegis to grant a bonus to the living conditions.
The idea that minor angels (and also faeries and magic beings, I suppose) help even out the fight against the demons is interesting.
Another thing to think is that diseases don't really cause a low living modifier, it's generally the other way around. On a rules level, this is accounted for by the possibility of an aging crisis inducing an illness.
So really, diseases caused by demons and diseases caused by living conditions would be two entirely different things.
Based on the following quote from A&A p36, I'd say demons are probably a relatively minor (but probably not insignificant) cause of diseases.
The humors really do
run the body; there are no cells, no pro-
teins, no DNA. Diseases are caused by
disruptions in the balance of humors;
rather than by bacteria or viruses, dis-
eases are caused by inappropriate diet,
bad smells, and unfavorable stars. Para-
sites are a symptom of disease, not a
cause of it, and so on.
Your average peasant living conditions modifier is 0, but -2 if living in a city. So it could be reasoned that this 2 points difference is the extra protection brought by the Aegis.
In the end, I would simply ignore this issue, assuming it is accounted by a multitude of factors not fully grasped by the rules. If you actually want to have a story where a disease is caused by a demon within the covenant, the possession rules can take care of that.
Where would the Aegis be involved in that comparison? Neither the average peasant, nor the average town-dweller live inside an Aegis.
I'd assume the living conditions of your average peasant living in your standard spring covenant is 0 (since mundanes only get a bonus in summer and autumn covenants).
And I'd be, of course, assuming the conditions for a mundane living in a covenant that happens to be inside a city is also 0 (unless said mundane spends most of his time in the city proper).
And I'd be positing that, on a covenant mostly w/o an Aegis, your living conditions would go down.
But again, this is just a thought. As I said, I'd just ignore the issue.
The thing is that the game follows medieval beliefs, which were not entirely consistent in terms of a modern perspective. Disease is caused by unclean spirits, but also by bad air, or imbalanced humors. Some of these inter-relate: bad air might well conjure unclean spirits (infernal or otherwise, after all an illness can be the source of a story!), and even modern mystical traditions will begin a spiritual cleaning of a space with a literal cleaning. One might argue that by allowing bad air or polluted water the covenant has invited unclean spirits in past their aegis, or that in this case the disease may be entirely worldly and simply an imbalance of humors due to an unhealthy environment. If the aegis is somehow better tuned to preventing these kinds of accidental invitations this might manifest as a healthy space bonus in the covenant description.
There's another section on there for covenfolk, who get a +1 bonus at the Summer/Autumn level. When it talks about peasants, that seems to just be the average peasant farmer, not specifically one at a covenant.
The -2 modifier from towns is because medieval towns are nasty. The reasons for that are different in Mythic Europe than in real life, since it'd be more rooted in how medieval scholars explained the difference, but the poor health effects of town living were very pronounced and well-observed at the time. In real life it was caused primarily by the packing together of lots of people and animals and poor sanitation practices like throwing shit into the drinking water. I'm sure in Mythic Europe, the things we'd think of as tertiary intensifiers of the problem become the main source of it, i.e. the disgusting smells, bad taste of the water, and lower variety of foods. But in any case, townspeople being less healthy and more prone to plague than peasant farmers is an established fact of medieval life.
Anyway, thanks everyone, I can see now that demons aren't the most important part of the equation! I guess I misjudged/misremembered how large that element was.