When you swim, some water is going to get into your mouth. For the most part, that is OK. In recreational pools chlorine is used to kill germs although it can take its sweet time killing some of them.

However, in 1999-2000, more than 2,000 recreational water illnesses (RWIs) and four deaths occurred because of water system failures in recreational pools. The most common RWI by far is diarrhea which affects thousands who accidentally swallow infected pool water.

This was 10 times the rate of the decade before, Michael Beach, epidemiologist at the Centers for Disease Control and Prevention in Atlanta, tells WebMD. Most swimming pool outbreaks go unrecognized and unreported. In the last decade, he estimates, pool outbreaks have affected 10,000 people.

On average people have about 0.14 grams of feces on their bottoms which, when rinsed off, can contaminate recreational water. People who have diarrhea have millions of germs. If a swimmer has diarrhea, he or she can contaminate water if he or she has an "accident" in the pool.

A particularly nasty customer that can be swimming alongside you is cryptosporidium, a parasite that laughs at chlorine and can cause diarrheal distress. The approximate disinfectant time in chlorinated water for this germ is nearly seven days. In the very young (say, that newborn you are "flying" through the water) or those with immune problems, crypto can cause severe debilitating illnesses. Some water parks have -- charmingly -- been described as "diarrhea farms."

"Crypto," says Beach, "can live in a chlorinated pool for days. Chlorine kills other organisms in a fraction of a second. This is a totally different beast."

Other unwelcome swim partners include E.coli, Giardia, and Shigella. "We see 2 million cases a year of Giardia," Beach says, speaking of other frequent fecal contaminants.

Americans, with their light-hearted "I don't swim in your toilet" signs, are pretty realistic -- if a little misguided -- about "group bathing." In May of 2004, Opinion Research Corporation (ORC) conducted a survey of nearly 1,000 people over the age of 18. Among the findings:

• About 60% said that it is "not likely at all" or "possible but not likely" that a person could get sick from pool water.
• Still, 88% agreed you should use soap and water after using the bathroom if you plan to jump back in the pool. Nearly 75% said they shower before going in.
• Nearly 94% said a "poop" accident should be reported immediately.
• 75% pointed the finger at diapered children (although Beach says adults who don't "wipe" thoroughly add 3 to 4 pounds of "solid" matter to the average water park).
• One-fifth said if you could smell the chlorine, the pool was safe (chlorine does kill germs, but some organisms die a slow death, lasting in a dangerous state for days). Also, a heavy odor means harmful chemicals have formed.
• One-fifth said a little urine never hurt anyone (urine, in fact, does not contain germs, but you can decide how you feel about that statement).

Who Me? Sick?

After relieving themselves (pun intended) of the above opinions, respondents to the ORC poll reported:

• 72% had red eyes
• 32% had ear infections
• 20% had a rash
• 10% had eye infections
• 6% got a respiratory or urinary tract infections
• 5% got a skin infection
• 4% experienced diarrhea

Michael W. Shannon, MD, is chief of emergency medicine at Children's Hospital in Boston. He says there seems to be a natural uptick in gastrointestinal illnesses in kids in summer, but he had never related it to gulping pool water before. "You could argue we are not looking for the pool as a cause," Shannon says. "Now I will be."

Eyeball Before You Cannonball

The Centers for Disease Control recommends using your senses to evaluate a pool before jumping in:

• Sight. The water should look clean, clear, and blue -- all the way to the bottom. You should be able to see the drain and the stripes on the bottom. Be sure the water is constantly lapping over the grills to be filtered.
• Touch. The sides of the pool should be smooth, not slippery or sticky. A handful of water should not stick to your hands.
• Smell. Chlorine should not have a strong smell. A strong chlorine-like odor can mean chloramines -- which are chemicals comprised of chlorine mixed with body oil, sweat, urine, saliva, lotions, and feces.
• Sound. Listen for pool-cleaning equipment.

How to Be a Good Pool Citizen

• Report problems to the pool manager; don't pretend it's "not you."
• Don't swim if you have diarrhea.
• Don't swallow water -- swim with your mouth closed, breathing only when your mouth is out of the water.
• Take a shower before swimming.
• Wash hands after changing a baby's diaper (and change kids in the bathroom not next to the pool).
• Take the kids on potty breaks, whether they ask or not. Check diapers often. If you hear, "I have to go," this can mean the child is already "going."
• Wash kids before swimming, especially their hind parts.
• Toddlers should wear special "swim diapers" and even these are not foolproof.

In the ORC poll, most people said it was the pool owner's responsibility to keep the pool clean. Only a fifth said it was the task of the swimmers or parents.

But think about it -- how many pool owners will you see gulping pool water this year in a public pool?

"Right now there is no quick fix for water-borne illnesses," Beach says. "Standards vary by state. All you can do is be realistic. Chlorine does not kill everything."

Don't stop swimming, he says, but be healthy and responsible about it.