Mold issues in Portugal during winter?

Well, two other places I've experienced mold and mildew are the English Lake District and the (U.S.A.) New Jersey shore. Do you have a formal diagnosis of mold allergy, or is your concern a vague fear of "Black Mold" from flooding-inundated buildings in the U.S.?

As someone who has been getting allergy shots (including Aspergillus Niger, a common mold but also the main ingredient of Beano OTC medication ... ) for 40 years, I can tell you that anyplace that has rain for 50% of the days (like most of the Eastern U.S. this summer!) will have mold spores in the air. The question for is is whether it clinically ... matters?

I'm not so concerned about mold spores generally in the air, but rather living and sleeping in a cottage/apartment/house that may have serious mold issues in the walls or rooms. I really want to stay in more out-of-the-way places, or at least smaller towns, and stay away from major hotels. Is this a wide-spread problem, or just a local "one-off" situation that I'll need to check?

There are definitely apartments that have mold issues due to the rain and lack of heat in a lot of places. You might want to inquire what type heat the apartment has. A large majority only have plug in oil radiators. While this works fairly well, having a fireplace or wall radiators would likely decrease the likelihood of mold problems. Also, a place with lots of sunlight would be better, and not ground floor.

I’ve not had problems myself but have friends who have fought the mold.

I would definitely ask the question of the person from whom you are renting.