Heat exhaustion
Extreme dehydration leads to heat exhaustion, a common malady for both competitive athletes and regular exercisers. If you're exercising on a hot day and start to feel headachy, nauseated, and/or a little woozy, as if you stood up too fast, stop immediately, rest in the shade, and drink a lot of water. The wooziness is caused by a drop in blood pressure, which resulted from blood going to the skin -- and not enough going to the rest of your body -- to try to regulate your temperature. Cooling down and resting allows your blood to go from your skin back into general circulation, and rehydrating by drinking a lot keeps your blood volume up (which increases your blood pressure, returning it to normal).

If you ignore these symptoms, you run the risk of heatstroke, a life-threatening shutdown of the body's thermo-regulating system. "Heatstroke occurs when you stop sweating, get the chills or faint," says Wells. "Then it's 911 time."

Swimmer's ear
This common summer malady is an infection in the outer ear canal caused by bacteria-rich water. It's easy to diagnose: The pain centers on the outer ear, and if you tug the top of your ear, it'll hurt. Your ear may also be swollen and red.