As always it's hard to comment on old build situations without actually see the situation but here's my tuppence worth:
Most likely your floors are suspended timber floors, built off a tassel wall with the underfloor void vented to the open air via the standard airbricks you see around the plinth. In these situations the ventilation tends to be inadequate due to inadequate or poorly located vents or obstruction of the vents. Often times you'll find the subfloor has an indequate finish (maybe no concrete- just soil or poor fill) and may be acting as a sump, i.e its level being lower than the surrounding ground level. I would recommend cutting an access point in the floor to see what's going on underneath.