Has been discussed here before.
As I wrote at the time, one thing I noticed in the UK was the fug in many homes.
A combination of warm air, moisture from shower, tumble drier, damp clothing, and no ventilation etc.
No windows open, central heating on full, folk wandering about in shirtsleeves.
No wonder mould thrives.
Oh, scrubbing with bleach, or specific mould cleaning stuff, helps. You dozy lazy shytebags.
We are having a new bathroom installed later this month. The chap frm the bathroom company who came to measure up the bathroom and do the plan for us asked if we wanted an extractor fan fitting. We asked what it would involve and he said that he didn't think we'd need one as there was no sign of any damp or mould. We told him that we always open the window and wipe the tiles down after a bath or shower and that during warmer weather the bathroom window was normally left open to provide extra ventilation for the landing and upper floor. He said that is why you don't have any problems with damp or mould. The bathroom is always cleaned regularly in any case.
He then proceeded to tell us some horror stories about some of the batherooms they have seen over the years. In some cases they had fitted a new bathroom and gone back several years later to fixed another problem only to find the bathroom grouting black with mould or mould on the ceiling. He said that the primary causes of damp and mould was not opening windows for ventilation and not cleaning the bathroom properly.
Like you said - open the windows, clean the bathroom regularly plus a little bleach now and then doesn't do any harm