Over time, mildew will build up anywhere that it is allowed. The discoloration can often be found first in the corners of your showers grout. It appears grey at first and then spreads and darkens. But, there is hope! Instead of dowsing your whole shower in bleach, which is very dangerous and costly, simply dampen some cotton balls with bleach and place them directly in the areas that the mildew is forming. This concentrates and localizes the bleach. Clean your shower as you normally would and, when finished, remove the cotton balls. The mildew should be gone. If not repeat with your normal cleaning routine until the discoloration is gone.