What They Are: A fungus that forms a red, itchy, raised circle on the skin (despite its name, it's not a worm at all). Fungi enjoy warm, moist areas and often appear in skin folds. (Fun fact: Jock itch is a kind of ringworm!)
Yuck Factor: Pretty low, once you get past the name. It sounds worse than it is -- and while it is contagious, at least there are no visible creatures to contend with.
How They Spread: Locker rooms, damp towels, wrestling or gymnastic mats, bathing suits or bathing caps, and even goggles can transmit ringworm. And it can spread to different parts of the body through scratching.
How You Know He's Got Them: An itchy (very itchy) red rash shaped like a ring is a dead giveaway, though ringworm can simply look red, scaly, and patchy like athlete's foot (a similar fungus) or just a regular old rash. Because it's not always clear, head to the doc; she'll scrape a little skin from the area to examine under a microscope.
What To Do: An over-the-counter antifungal cream will usually work. But if the infection is on the scalp or is particularly stubborn (lasting more than two weeks), your doctor will likely order a fungus-killing prescription. You'll want to wash all towels, sheets, bathing suits, and whatnot. One wash does the trick.