It may 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.
"Molds can gradually destroy the things they grow on," the Environmental Protection Agency says in a pamphlet for consumers on its website. "You can prevent damage to your home and furnishings, save money and avoid potential health problems by controlling moisture and eliminating mold growth."
It's important to do both — control the moisture and clean up the mold — to keep it from growing back, the EPA says.
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.
Frequently, the bathroom is the first place that a homeowner or apartment dweller might notice mildew.
"A lot of people take a shower, get out of the shower" and go on their way, Collier said. "Basically, they've left all this water on the walls and floor of the shower and surprise, surprise, the bathroom is damp and you find mildew growing on the grout."