Anne painted her bathroom with latex paint and then started to get mould and mildew where she never had any before. She is wondering what she did wrong?
Mould is caused by the prolonged presence of condensation. Condensation is caused by a combination of high humidity and low temperatures. If you make some change in a room that either lowers the temperature or raises the humidity, and you were close to the threshold of condensation before, you could have tipped the scales in favour of creating condensation.
Assuming that Anne really changed nothing else in her bathroom but the paint, changing from an oil paint to a latex paint could make that slight difference, given that latex paint absorbs moisture a little and could hold it longer.
Increasing the temperature of the ceiling by adding more insulation in the attic above, or reducing the humidity by installing a stronger exhaust fan, could stop the problem.
If you want to be sure that addition of latex paint to a bathroom doesn't make that slight change that gives you a problem, you can purchase mildew resistant paint, or purchase an additive, like the one you see in the photo, that will resist mildew growth and probably counteract the slight change you have made to your bathroom. But before you do this, read this recent article on "antimicrobials" and their side effects.
Personally I would check out the insulation and the ventilation as a more sure-fire solution.