Mildew is a fuzzy, usually grayish-white fungus that grows in damp places. You might notice mildew on a wet towel left crumpled on the bathroom floor, on the grout between shower tiles, or on leaves in a humid garden. It has a distinctive musty smell that tells you something has stayed wet too long.

Mildew thrives in moisture and warmth, which is why it appears on things that don't dry properly. A beach towel tossed into a gym bag after swimming, a basement with poor air circulation, or a tent packed away while still damp can all develop mildew. The fungus spreads by releasing tiny spores into the air, so a small patch can quickly become a bigger problem.

While mildew itself is mostly just unpleasant and smelly, it's related to mold, which can be more serious. Preventing mildew is simple: keep things dry and give damp areas good airflow. If mildew does appear, cleaning it quickly and drying the area thoroughly usually solves the problem.