Recently we stayed overnight in Glasgow, in the west end by Kelvingrove Park. Several accommodations recommended by Rick Steves were booked so I made a reservation for a nearby hotel with high Trip Advisor ratings. We booked about 4 months in advance. When we arrived we asked to see the room and were led down to a basement room that smelled strongly of mildew. Not wanting to be 'ugly Americans' we were unskilled at being assertive about our displeasure and just asked if there were any upgrades available and were told no. In addition, they had a Saniflo system toilet, which was noisy and difficult to flush. We could hear others' toilets flushing frequently. We slept poorly and at 1:00 A.M. I hatched plans to ditch the hotel (we'd booked for three nights) that morning and drive up to the Highlands, as my husband ended up being much more skilled driving on the left than we'd expected. Although we checked out by 8 am, we were only refunded the third night. We were charged 261 pounds for the first night! We were tired, didn't have access to our original booking information and decided to just eat the cost and go.
I regret that we didn't immediately terminate our reservation upon smelling that room, and just not stay there at all. My husband has a mold allergy and felt ill that evening, and a couple of days later we both ended up with dry coughs, stuffy & bloody noses, and fevers. Three days later upon arrival at home we went right to the doctor's office from the airport. Now we both have unusual nasal symptoms (white spots on the inside of our septums) and I still have a fever. Yes, it might just be regular colds or sinus infections, but at minimum the irritation from the mildew in that room was definitely a contributing factor.
You have only one life, and your vacations are precious. If your room is awful, consider not staying at all and booking elsewhere. It's uncomfortable to do so (especially if the front desk staff are very kind), but if you wouldn't normally sleep in a mildewy basement bedroom at home, why would you accept this on a vacation and risk illness during your trip? I'm sure there are ways to be assertive and polite, even though it's hard when you are exhausted and feel like you are trapped with no options. Because we simply left the hotel and didn't directly complain about the mildew, I don't want to name the hotel. But, this tale of woe should apply to any room with cleanliness issues. I'd be interested to hear others' experiences and what they've done. In theory, of course you'd think you would be assertive, but when you are in the situation, it's more difficult. Note: we did use Covid tests I'd brought with me (negative) and the PCR tests when we got home were negative as well.