Draughts :-) It's an old place, it doesn't seal. That and sleeping in rooms with the window open just a crack as much as possible. Mould tends to hit when people obsessively seal everything up.
I still have a condensation problem in a few specific places I have to keep an eye on. It's not general.
I think he's asking about the common trap people seem to fall into where, trying to keep heat in, they shut down air changes too far. Human breath puts a surprising amount of moisture into the air, and cooking can too. If you let that build up and have a cold wall it can build up a film of damp very quickly and if it's left damp and cool it can encourage mould. That kind of bad air quality is far worse than a bit of a chill.
I still have a condensation problem in a few specific places I have to keep an eye on. It's not general.