The biggest culprit seems to be a parasite called Cryptosporidium. It multiplies in the gastrointestinal tract and can escape through a leaky diaper or other poolside accident and, if swallowed, can cause a distressing bout of diarrhea. Cryptosporidium is highly chlorine resistant and can survive in pool water for days. E. coli, another escapee from the gut known to cause outbreaks of stomach malaise, is more sensitive to chlorine and can usually be kept in check with careful pool maintenance.
As a first precaution, take a close look at the water before you jump in. Its color and texture are good indicators of its cleanliness. It should be clear enough for you to see through at least 10 ft. of water and distinguish objects such as a metal grating on the bottom of the pool. Foamy or bubbling water along the pool's edge is a sign of potential trouble; it typically represents excessive organic matter, such as pollen or bacteria. If the water looks clear enough to enter, the next line of defense is to keep your mouth shut. As we swim, many of us unknowingly swallow water--along with any microbes that might be in it.
The best defense is a good offense, fighting the contaminants at their source. Be wary of babies wearing diapers. Parents should be changing them frequently and cleaning up thoroughly, even if the little ones are wearing so-called waterproof diapers. Parents should be careful where they change a diaper, doing it as far away from the pool as possible. And whether you've changed a baby or gone to the bathroom yourself, you should always wash your hands with soap and water before re-entering a pool.
Chlorine is still the most popular weapon against contaminated pool water. Home pools should be tested at least once a day; many public pools are checked every one to three hours, depending on the number of swimmers. Because bugs love warmth, heated pools and Jacuzzis require more rigorous testing and higher levels of chlorine. If you are feeling particularly vigilant, you might pick up one of those $10 home-pool testing kits that measure the water's pH level to make sure it's in a safe range. And then enjoy your swim!
Dr. Ian appears on NBC's Today show. firstname.lastname@example.org