We’re all familiar with colorful spots of something growing in our showers and in other dark, damp areas in our homes, but you may not know what to call it. Is it mold, or is it mildew? What is the difference between the two, anyway?

Both terms refer to fungus, but as it happens, it’s a squares-versus-rectangles situation. Mildew is a type of mold. The term typically describes fungi that grows flat, on surfaces like the walls of your shower or window sills. There are also several types of mildew that are specific to plants—powdery mildew and downy mildew are parasites that grow on certain trees, flowers, and crops, for example. While mold might be a colorful green or black, mildew is typically white.

The word mildew originally came from honeydew, a term for sticky secretions aphids and other insects leave on plants, which people used to think came from the sky, like dew. Eventually, the word came to refer to the mold caused by the fungi that fed on these secretions.

Powdery mildew on maple leavesiStock.com/kazakovmaksim

Most of the household growths we refer to as mold belong to just a few families of fungi species. According to the CDC, the most common indoor molds are Cladosporium, Penicillium, Aspergillus, and Alternaria. Household molds can be a variety of colors, from orange-brown to green to gray to black. (Note that not all mold that is black in color is the more toxic species we call “black mold,” or Stachybotrys.) In contrast to the powdery texture of mildew, molds are typically fuzzy or slimy.

In nature, mold can play an important role in the ecosystem, breaking down dead plants and leaves. In your house, those decomposition abilities aren’t quite so welcome. Mold spores fly through the air, and when they land in moist places, they start to grow—whether that’s on food, your ceiling, paper products, wood, carpet, leather, or elsewhere around your house—and in the process, destroy whatever they're growing on. Unlike mildew, most molds grow down into the surface of its habitat, making them more difficult to remove. In porous materials, mold grows into all the empty crevices, which is why it is often impossible to remove all the mold from ceiling tiles (or soft foods like bread).

Mold growing in a Nashville home following a floodMartin Grube, FEMA // Public Domain

Getting rid of the unsightly growth in your damp bathroom is more than just a matter of aesthetics. Indoor mold can cause allergic reactions, such as a stuffy nose or itchy eyes, and can lead to infections for people with compromised immune systems. Some people are more sensitive to mold than others, and may experience more symptoms when exposed to it. Generally speaking, though, mold spores are everywhere, so you’re never going to live a totally mold-free life. Spores will come into your home through windows, doorways, ventilation and climate control systems, and via your clothing, shoes, and pets.

But there’s only one way to effectively inhibit mold growth at home: Get rid of the moisture. That means fixing leaks, getting better ventilation, and possibly running a dehumidifier, according to the CDC’s recommendations on mold.

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