Once a bathroom has mildew on the ceiling and or walls, the mildew must be eliminated, not just wiped away. Often homeowners, especially those who dislike painting, will try and cut corners on preparation and end up going over their work soon after completing it. When mildew is present, you can't miss it. The black growth grows on wallpaper, paint, caulk or anywhere it can. Either use specially prepared cleaning products supplied at paint, hardware or home improvement stores, or mix up your own. Take household bleach and mix it 50/50 with water and with a pail, sponge, and rubber gloves, you're ready to go. Wipe the solution on the mildewed surface and let it sit for ten to fifteen minutes.  Rinse it off, let it dry and continue. If your bathroom does not have good ventilation, it is recommended you use paint made for kitchens and baths. Zinnsser sells Perma-White in satin finish or semi-gloss finish, which can also be tinted to different colors. Benjamin Moore makes Kitchen and Bath which does essentially the same thing. These paints are formulated to absorb moisture and resist mildew formation. They require a 2 coat application for the intended results. The shower must not be used for 24 hours, to allow for curing of the resins and polymers. Permawhite was used early on in inner city tenement buildings, where people lived in one room dwellings, and found to be effective and successful. For people who still like wallpaper in the bath and kitchen, it depends on the backing of the wallpaper as to whether you must replace or simply clean off the mildew, if present. Heavy fabric-backed vinyl may be able to be wiped down as above and the installation of a vented fan in the ceiling or wall could be the end of the problem. Although, sometimes the brown vinyl adhesive used to install the wallcovering can "feed" the mildew growth. Thinner papers with coatings or paper backings can also be mildew-prone. Often it is necessary to replace the paper and start over if there is too much dampness. Just bear in mind that mildew will return if not killed with the right cleaner, just because you wipe it away and it looks as if it's gone, doesn't mean it is! Good luck!