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Germs in the Pool: A Survival Guide for Swimmers

Chlorine kills most pathogens, but swimmers need to protect themselves from other bacteria that lurk below surface

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Although jumping in the pool is a great way to escape the heat and get some exercise, be aware there are plenty of disease-causing germs lurking below the water's surface.

Between 2015 and 2019 the Centers for Disease Control and Prevention (CDC) recorded 208 outbreaks of illness linked to pools and hot tubs that resulted in 3,646 infections, 286 hospitalizations and 13 deaths, and experts say those numbers could be much higher.

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Nearly all of the recorded outbreaks (96 percent) were associated with public pools, hot tubs or water playgrounds. Hotels and resorts accounted for 34 percent of outbreaks, with the majority of them originating in hot tubs (70 percent) versus pools. And most outbreaks occurred in the months of June, July and August.

“With public pools, you have more people with more germs coming into that water,” said Michele Hlavsa, chief of the CDC’s Healthy Swimming Program. “So, they are more likely to have an outbreak versus if it's just one family using the pool — they're bringing in less germs.”

Chlorine can’t kill all germs

  • When someone pees in the pool, it mixes with the chlorine, leaving less chemical available to kill germs.
  • And chlorine doesn’t kill germs right away. While it kills most germs within minutes, some can live in a properly chlorinated pool for days.
  • When you swim in a pool, you're sharing the water with everyone else in it. If a person with diarrhea is infected with crypto or giardia, it takes only 10 or fewer germs to cause infections.
  • Just one person can contaminate an entire pool.
  • Even swallowing a small amount of contaminated water can make you sick for up to three weeks.

Source: CDC

Most common diarrhea- and pneumonia-causing germs

Cryptosporidium: The leading cause of recreational water-related outbreaks originated from cryptosporidium (also known as crypto), a parasite that can cause a gastrointestinal illness such as diarrhea, the CDC report found.

“Once [crypto] gets into the pool water and it's exposed to chlorine levels that you would expect to see in a well-operated pool, it can survive for more than seven days,” said Hlavsa, who also coauthored the CDC report. If swallowed, the parasite can cause diarrhea that lasts more than three days — and this can be especially dangerous for older adults, she added.

Legionella: A bacterium called legionella was the next most common cause of outbreaks and the source of all 13 recorded deaths between 2015 and 2019. It can cause a severe type of pneumonia, called Legionnaires’ disease, or a less serious illness called Pontiac fever, which can cause flu-like symptoms. People 50 and older, current or former smokers and those with weakened immune systems are more likely to develop a lung infection from the bacteria, according to the CDC.

“When I think legionella, I think hot tubs,” Hlavsa said. “And when the hot tubs are jetting that air out, if there’s legionella in the hot tub, the droplets can be inhaled.”

Her advice? Older adults may want to avoid hot tubs completely. “Not only not getting in, but staying away from the area of the hot tub,” she said.

Other common ailments linked to pools and hot tubs are skin rashes, ear pain, cough or congestion, eye pain and other infections.

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If you think you’ve gotten sick from a pool, seek medical attention and notify your local health department. If health officials start making connections between people who are getting infected, they may launch an investigation to identify a potential outbreak, Hlavsa said.

​Top 5 causes of outbreaks​

  • Cryptosporidium (parasite): 37 percent
  • Legionella (bacterium): 31 percent
  • E. coli (bacterium): 2 percent
  • Giardia (parasite): 1 percent​
  • Norovirus (virus): 1 percent​​

10 ways to stay safe from a pool-related illness

1. Stay out of the water if you have diarrhea. If you have been diagnosed with cryptosporidiosis, the illness caused by the parasite, don’t go back into the water until two weeks after diarrhea has completely stopped.

2. Shower before you swim. Rinsing off in the shower for 1 minute removes most of the dirt or anything else on your body that uses up pool chemicals to kill germs. Most people have about 0.14 grams of feces — comparable to a few grains of sand — on their body at any given time, according to the CDC.

3. Don’t swallow the water. The number one way you can protect yourself from crypto is to avoid swallowing the water, Hlavsa said. Chlorine kills most germs within minutes; it doesn't kill germs instantly.

4. Have kids? Take frequent bathroom breaks. For those who are very little, check diapers every hour and change them away from the water to keep germs from contaminating it.

5. Beware of red eyes and a strong “chlorine” smell. The smell comes from a chemical irritant called chloramine that occurs when chlorine combines with what washes off swimmers’ bodies. Chloramine also causes red, blood-shot eyes. An abundance of chloramine likely means there is an excess of urine, feces or sweat in the pool. It also means there’s less chlorine remaining in the water to kill germs.



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6. Protect open cuts or wounds. It’s best to stay out of the water, especially if the cut or wound is the result of a recent surgery or piercing. If you do go in, use waterproof bandages to completely cover the cut or wound.

7. Mind your ears. Dry ears thoroughly after swimming or wear a bathing cap or ear plugs to prevent swimmer’s ear, an infection in the outer ear canal. The bacterial infection is more common in children.

8. Shower after with soap. After getting out of water for the day, remove your swimsuit and lather up to kill any germs clinging to your skin. Wash your swimsuit, too — this prevents “hot tub rash,” an itchy red bump that can fill with pus around hair follicles.

9. Conduct your own inspection. If it doesn’t meet proper criteria, talk to the pool operator or go to your local health department to make sure the pool or hot tub is operating properly:

  • Make sure the drain at the bottom of the deep end of the pool is visible, secured and in good condition.
  • Use test strips that can be purchased at a pool supply or hardware store to check the chlorine or bromine levels and pH in the water.
    • Chlorine should be at least 1 part per million (ppm) in pools and at least 3 ppm in hot tubs.
    • Bromine, an alternative to chlorine, should read at least 3 ppm in pools and 4 ppm in hot tubs.
    • The pH level represents how effectively germs are killed and should be between 7.2 and 7.8.

10. Do your part. “If swimmers and caregivers of young swimmers are doing their part — not swimming with diarrhea, not swallowing the water — I think we can all have a great summer in and around the pool this year,” Hlavsa said. “It's just going to take all of us doing that together.”

Editor's note: This article, originally published May 25, 2021, has been updated with additional information.

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