Bathrooms are a very common place to get damp and mould from condensation due to all the large amount of moisture that is produced when you shower or take a bath. This moisture then evaporates into the air increasing the humidity levels. If the room is not ventilated enough this can mean that surfaces take a long time to dry out and the damp air is left in the room. This damp air will eventually settle on surfaces and cause mould to grow.
Often all that is needed is to open a window once you have finished showering or having a bath and leaving it long enough to dry the room out. In the winter, having your heating on can also cause a lot of condensation around the window where the cold air hits the warmer air. Again, opening the windows a little and if possibly drying off damp surfaces and window ledges can avoid the mould creeping in.
Whilst small amounts of mould are harmless, if left untreated, prolonged exposure to mould can cause skin allergies and respiratory conditions to worsen, situations you will want to avoid. If you can open windows at opposite ends of your property and prop doors open, this is the best way to get air to circulate efficiently through your home.