It might start with a couple of small dark spots on the grout where the shower stall meets the tiled wall. A few days later, they multiply.
'For most people, it's just kind of an ugly pain. Their bathroom gets these black stains, ' said Ken Collier, editor in chief of The Family Handyman.
For others, though, 'it's an allergen, like cat hair, dog hair, ' he said. If it's not removed, it can result in respiratory problems or other allergy symptoms, according to the Centers for Disease Control and Prevention.
Mold and mildew, which the Federal Emergency Management Agency calls 'mold in early stage, ' can grow on a wide range of surfaces as long as moisture is present, and they can give your house a musty smell.
'It's an indication of dampness, like in bathrooms that are not ventilated very well, ' said Kathie Birenbaum, who works in households at Strosniders Hardware Store in Bethesda, Md.
It's important to control the moisture and clean up the mold to keep it from growing back, the Environmental Protection Agency says on its website.
Cleaning large areas - more than 10 square feet, the EPA says - might require a contractor trained in mold removal. For smaller areas, it can be a do-it-yourself job.
'The bathroom fan is a big part of keeping the moisture level down in the bathroom, ' he said. 'If there isn't one, put one in. '
Opening the bathroom window also helps, as does more frequent cleaning, according to the EPA.
There are many products available for cleaning mildew and mold, but homeowners can mix their own. A bleach solution that CDC says should be 'no more than 1 cup of bleach in 1 gallon of water ' can be used to clean and kill mildew.
If you can't get rid of the mildew with bleach, you might have to dig out the grout and replace it, Collier said.