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Well-maintained pool water kills almost everything quickly. Almost.
Cryptosporidium, a parasite spread in feces, has a tough outer shell that chlorine struggles to penetrate. CDC puts it plainly: at recommended chlorine concentrations, free available chlorine inactivates most pathogens within minutes, while extremely chlorine-tolerant Cryptosporidium can survive for more than seven days.
That single fact drives everything below.
Key figures
- more than 7 days Cryptosporidium survival at CDC-recommended chlorine levels
- two weeks Time out of the pool after diarrhea stops
Why this matters for a home pool
Public pool outbreaks make the news, but the transmission route is the same in a backyard: one person with a recent bout of diarrhea, a small accident that nobody notices, and water that looks perfectly clear.
For context on scale, CDC recorded 493 outbreaks associated with treated recreational water during 2000 to 2014, causing at least 27,219 illnesses and eight deaths.
Anyone who has had diarrhea should stay out of the water for two weeks after symptoms stop. Crypto keeps shedding after a person feels better, and this is the rule that prevents most transmission.
What to do after a fecal accident
A solid stool is the manageable case:
- Everyone out of the water.
- Remove the stool with a net and bin it. Do not vacuum it, which spreads it.
- Clean and disinfect the net.
- Check that free chlorine and pH are at recommended levels, and let the water circulate for at least 30 minutes before anyone returns.
Diarrhea is the serious case, because it may contain Crypto:
- Everyone out, and keep them out.
- Remove as much as possible, as above.
- Hyperchlorinate. CDC publishes the concentration and contact time needed to inactivate Crypto, and it is a much higher chlorine level held for hours.
- Keep the filter running throughout, then clean or backwash the filter afterwards.
- Let the chlorine fall back to the normal range before anyone swims.
Follow the CDC’s current published values for the hyperchlorination step rather than a number remembered from a forum: the level and the time depend on each other, and stabiliser in the water changes both.
Prevention, which is entirely behavioural
- Nobody swims with diarrhea, or within two weeks of it stopping.
- Everyone showers before getting in. It removes most of what chlorine otherwise has to deal with.
- Bathroom breaks every hour for children, and check nappies away from the pool.
- Swim nappies are not waterproof. They slow a release, they do not contain it.
- Do not swallow the water, which sounds obvious and is the actual route of infection.
If people get ill
Watery diarrhea starting a few days to a couple of weeks after swimming is the classic pattern. It matters for two reasons: the person needs care and possibly testing, and everyone else who used that water needs to know. Tell your doctor that there was pool exposure, because Crypto is not found on a standard stool test unless it is specifically requested.
Related reading: The Only Two Numbers Your Pool Water Needs and Stabilizer Protects Chlorine and Then Handcuffs It.
Questions people ask
Can you get sick from a properly chlorinated pool?
Yes, from Cryptosporidium. CDC states it can survive more than seven days at recommended chlorine levels, while most other germs die within minutes.
What do I do if there is poop in the pool?
Clear the pool, remove the stool with a net, disinfect the net, and confirm chlorine and pH. For diarrhea, follow the CDC hyperchlorination procedure, which needs a much higher level held for hours.
How long after diarrhea can someone swim?
Two weeks after symptoms stop, because shedding continues after a person feels well.
Do swim diapers stop this?
No. They contain solids briefly and do nothing for diarrhea. They are not a substitute for keeping unwell children out of the water.
Where these figures come from
- Outbreaks Associated with Treated Recreational Water, United States, 2000 to 2014 Morbidity and Mortality Weekly Report, Centers for Disease Control and Prevention · cdc.gov
- Home Pool and Hot Tub Water Treatment and Testing Centers for Disease Control and Prevention · cdc.gov
- Cryptosporidium Centers for Disease Control and Prevention · cdc.gov
Figures last checked September 2026.
