Some of this suggests a more communal style of living. Villages and towns would have a few longhouses or apartments that hold most of the people, and individual houses would be rare. Why? Because if you have one or two people who can cast a few spells, then they aren't going to run from house to house to start fires or freshen pantries. Instead, there would be a more centralized source of fire and fresh food and water, and people living around that. In larger towns where there are more spellcasters, then there would be multiple such centralized areas, but they'd still be communal. It'd end up being a type of clan, with the casters being as important, or even more important, than the non-casting clan head--assuming that they aren't the head of the clan, and this isn't a magiocracy.
If lots of people can cast a few spells (even if they're only cantrips and not even 1st-level spells), then you can decentralize and have more individual households. But even these may have multiple related families living together than what we consider a typical D&D household to be like (which, lets face it, is very modern in style).
I will argue that healers may not have a better understanding of what disease is. I don't think that the spells are designed to tell the caster both what the disease is and what caused it (i.e., "your stomach ailment was actually the result of [food poisoning from bacteria-laden undercooked chicken/a parasite from contaminated water/stomach cancer], but don't worry; it's cured now") and I think it's entirely likely that casters would just assume that the illness was caused by whatever the popular theory is--which could be "bad air" or "a fairy cursed you." This could actually be a reason why people don't get cured willy-nilly; if some illnesses are believed to have been caused by "sinful" actions, then clerics and druids may choose to withhold healing from them, because the Gods Have Spoken.