Mildewy hotel room

I sympathize. I had a similar experience on BC west coast. I did know it was a wooden structure but totally underestimated the mildew and mold. I took an antihistamine right away and donned a N95 the entire night and while in the room.
We did try to change the room, and they did refund second night.
If you have allergies, never hesitate to speak up right away. You don't need to be confrontational, but even Rick says to check your room before accepting it. If I have a complaint, I consider what I want to rectify the problem.
I would also send an email to the manager to explain the issue so they can fix the room. Of course, I would also give an honest TripAdvisor review. But in the future, stand up right away...with a smile and patience.

I've had a few rooms over the years that had terrible smells--mildew, curry, fish. I never accepted any of them. One hotel offered me two rooms that had bad smells. In Paris. I didn't care about being seen as an "ugly American." I'm a paying guest. They found me a room without a smell.

You would not have been an “ugly American” for refusing a room that was unsuitable to stay in. The hotel was at fault for offering that room to any guest. You should reveal the name of the hotel. What is your reason for protecting them when they exposed you to health risks?

Even though you do not want to name the hotel here, I suggest doing a Trip Advisor review noting the mildew smell in the basement room so others are aware and can ask not to be put in rooms on the lower level.

Although I’m sure you had your reasons for not naming the hotel, I wish you would reconsider as I’m sure it would be useful to prevent others being treated so badly and actually getting ill like you did. I hope you’re recovering well.

To those that encouraged me to reconsider doing a review on TripAdvisor - I have considered and explored that, please recall that the hotel has a 4.5 rating, with lots of reviews, so the bad ones are buried. I myself would never have found my own bad review. That's the whole point of my rambling post, I guess, that even if you book based on a good rating, if the room is unsuitable, boom! you do something about it. I can make excuses for being an infrequent traveler (and zero travel the last 3.5 years), being jet-lagged, not feeling confident about city driving on the left, etc., but in the end, we should have just ditched then and there. The awfulness of the experience did force us to make a more dramatic decision to alter a third of our planned itinerary for a much better trip, so I guess that's the sunny side of things.

I felt a bit vulnerable posting my failure on the forum, but felt a duty to help others. In my opinion, reminding infrequent travelers that they have the right to a clean smelling room and to take action immediately, is more effective than crossing my fingers that someone will read the bad reviews of a "4.5" hotel before booking. Better to equip newbies like me with reminders to politely assert themselves wherever they go, in my opinion.

Mina, thank you again for sharing your experience. I know what you mean about feeling vulnerable sharing with others when you felt you have made a mistake. What I hate though it that obviously it’s not your fault - the hotel should never be putting guests in a room like that in the first place !!

Mina, thanks for going outside your comfort zone to shsre your experience. It is useful for both newbies and experienced travelers.
May I offer another strategy for future hotel bookings? As a solo older woman traveler, depending where in the world, we sometimes are overlooked, or treated a little poorly as 'just a woman'. On the other hand, my white hair has gained me respect and politeness. But when I book a hotel I always ask for something in the booking....joined twin beds, a window room, a quiet room away from the elevator or stairs, or if elevators, a high room, guaranteed late arrival, early check in.....whatever. I acknowledge no request is guaranteed, but would appreciate the consideration. I also make a point of reconfirming all bookings. The point of these are not to be considered picky but to be considered at all. One looks more like a decerning client than an easy pushover. It also catches any overbookings or problems if you arrive and find out they don't have your booking.
As to losing your poor review, at least on the PC or web version of TripAdvisor, you can filter reviews by 2, 3 , 4 star reviews, the month of travel or the type of travel (solo, family, etc). That's also a tip for you when reviewing in the future, and your voice and experience will be read by those that filter.
Glad there was a silver lining afrerwards, and hope you are feeling better.