Dermatologists agree on one thing: clean, dry underwear is your first line of defense against irritation, odor, and infections like jock itch. But wearing the same pair for a second day, by itself, usually won’t trigger a fungal outbreak. The real danger comes when moisture, friction, and time combine. If you sweat heavily, work out, or live in a hot climate, that damp fabric traps heat and microbes close to the skin, breaking down its natural barrier.