Swimming Pool Illness Outbreaks: U.S. Registry of CDC-Reported Outbreaks, 2009 to 2023

Registry of swimming pool illness outbreaks, 2019 to 2023 (newest first)

The registry lists the 156 treated recreational water outbreaks with first illness from 2019 to 2023 in CDC’s public National Outbreak Reporting System file, newest first. Each entry gives onset month, state of exposure, venue, setting, cause as reported, etiology status, cases, hospitalizations and deaths. The full 1,085-entry registry from 1978 to 2023 is available as a download.

How many swimming pool illness outbreaks happen each year in the U.S.

U.S. health departments reported an average of 37.3 treated recreational water outbreaks and 569 outbreak cases a year from 2009 to 2023. The series is uneven: large Cryptosporidium years (2016) and the 2020 pandemic dip move it more than any trend, and the most recent years are still filling in.

In 2016, 58 U.S. outbreaks and 2,020 cases were reported, the highest case count since 2008; Cryptosporidium accounted for 41 of those outbreaks and 1,862 of the cases (CDC NORS).

In 2020, the U.S. file shows 9 outbreaks and 36 cases, 80.7% fewer outbreaks than the 2015 to 2019 annual mean (PC&S analysis of CDC NORS). CDC’s 2021 summary suggested that jurisdictions may have changed outbreak reporting during the pandemic because resources were diverted to the COVID-19 response (CDC WBDOSS 2021 summary).

From 2019 to 2023 the United States averaged 31.2 outbreaks and 304 cases a year, against 37.3 outbreaks and 569 cases a year across 2009 to 2023 (PC&S analysis of CDC NORS).

Which germs and chemicals cause pool, hot tub and splash pad outbreaks

Two organisms dominate U.S. treated recreational water outbreaks: Cryptosporidium causes most cases, Legionella causes most hospitalizations and every recorded death. Chemical exposures are the third-largest group by outbreak count, most of them at pools.

Cryptosporidium caused 186 U.S. outbreaks (33.2%) and 4,944 cases (57.9%) from 2009 to 2023 (PC&S analysis of CDC NORS). Counting only laboratory-confirmed Cryptosporidium outbreaks, the share of cases is 56.8%; excluding the 2016 peak year, it is 47.6% (PC&S analysis of CDC NORS).

Legionella caused 190 U.S. outbreaks (33.9%) but only 940 cases (11.0%) from 2009 to 2023, because most Legionella outbreaks are small; it accounted for 413 hospitalizations (59.9%) and all 22 deaths (PC&S analysis of CDC NORS).

CDC’s 2015 to 2019 review reached the same pattern: Cryptosporidium caused 76 outbreaks and 2,492 cases, and Legionella caused 65 outbreaks and all 13 deaths (CDC MMWR, 2021). For 2000 to 2014, CDC attributed 89% of cases in confirmed infectious outbreaks to Cryptosporidium (CDC MMWR, 2018). Outside the United States, a 2026 review of 23 peer-reviewed articles on pool outbreaks from 2020 to 2025 also found Cryptosporidium and Legionella the most frequently reported pathogens (Steinberg et al., 2026).

Chemical exposures caused 68 U.S. outbreaks and 1,293 cases (15.1%) from 2009 to 2023, 54 of them at pools (PC&S analysis of CDC NORS). CDC notes chemical outbreaks are likely under-reported because hazardous materials responders and infectious disease staff may not share reports (CDC MMWR, 2021). Emergency department data on pool chemical injuries are covered in PC&S pool injury statistics.

Pools, hot tubs and splash pads: which venues cause which outbreaks

Pools account for most outbreak cases; hot tubs account for most outbreak deaths. Pool outbreaks are mostly Cryptosporidium and chemical; hot tub outbreaks are almost entirely Legionella and Pseudomonas, and no hot tub only outbreak in 2009 to 2023 was caused by Cryptosporidium.

Hot tubs or spas alone were the venue in 176 U.S. outbreaks (31.4%) but only 916 cases (10.7%) from 2009 to 2023 (PC&S analysis of CDC NORS). Those hot tub outbreaks produced 310 hospitalizations and 20 of the 22 deaths (CDC NORS).

Legionella caused 147 of the 176 hot tub outbreaks (83.5%) and Pseudomonas caused 18 (10.2%), together 93.8% of hot tub outbreaks and 88.2% of hot tub outbreak cases (PC&S analysis of CDC NORS).

Splash pads or water playgrounds alone were the venue in 24 U.S. outbreaks and 279 cases from 2009 to 2023; Cryptosporidium caused 16 of them (CDC NORS). CDC’s splash pad review counted 60 outbreaks and 10,611 cases from 1997 to 2022, with Cryptosporidium in 40 outbreaks and 9,622 cases (CDC MMWR, December 2024). In June 2021, a Kansas wildlife park splash pad was linked to a Shigella outbreak with 21 cases among study respondents and a separate norovirus outbreak with six (CDC MMWR, 2022).

When outbreaks start: month of first illness

Treated recreational water outbreaks have a summer peak driven by Cryptosporidium, and a smaller March peak in which Legionella, Pseudomonas and other causes outnumber Cryptosporidium.

277 of 560 U.S. outbreaks (49.5%) from 2009 to 2023 began in June, July or August; July had the most, with 108 (PC&S analysis of CDC NORS).

158 of 190 U.S. outbreaks involving Cryptosporidium (83.2%) began in June, July or August from 2009 to 2023 (PC&S analysis of CDC NORS). Local swim season length by city is covered in PC&S pool season length by city.

March was a secondary peak with 56 U.S. outbreaks from 2009 to 2023 (CDC NORS). 62 of 176 hot tub only outbreaks (35.2%) began from December to March (PC&S analysis of CDC NORS). CDC reported the same bimodal pattern for 2000 to 2014, with 56% of outbreaks starting in June to August and 9% in March (CDC MMWR, 2018).

CDC’s report on a March 2023 Pseudomonas aeruginosa outbreak at a Maine hotel pool, where 23 of 35 responding guests (66%) developed ear pain, rash, or foot or hand pain, notes that hotel Pseudomonas outbreaks predominantly occur during January to April (CDC MMWR, 2024).

Where outbreaks are reported: treated recreational water outbreaks by state

Forty-four states, the District of Columbia and Puerto Rico reported at least one treated recreational water outbreak from 2009 to 2023. State totals mostly reflect how actively health departments investigate and report, so they rank surveillance as much as risk.

Florida reported 79 outbreaks from 2009 to 2023, 56 of them Legionella; Minnesota reported 64 and Ohio 51, with Cryptosporidium the most common cause in both (CDC NORS). Ohio’s 51 outbreaks produced 1,341 cases, the most of any state, including 638 from the July 2016 Cryptosporidium outbreak (CDC NORS).

Why CDC outbreak counts change after publication

NORS is a live system. Counts in CDC’s first published summaries are lower than the same years in the October 5, 2026 download of the file, by about 12% for 2015 to 2019 and 36% for 2021. All counts are also a small fraction of modeled illness.

CDC’s May 2021 MMWR counted 208 treated recreational water outbreaks and 3,646 cases for 2015 to 2019, using reports filed by February 4, 2021 (CDC MMWR, 2021). The current file lists 233 outbreaks and 4,053 cases for the same years, 12.0% and 11.2% more (PC&S analysis of CDC NORS).

CDC’s 2021 summary counted 22 outbreaks and 219 cases using data as of January 19, 2023 (CDC WBDOSS 2021 summary). The current file lists 30 outbreaks and 349 cases for 2021, 36.4% and 59.4% more (PC&S analysis of CDC NORS). The 2000 to 2014 count differs by only 0.8% (PC&S analysis of CDC NORS).

CDC modeled 5.61 million illnesses linked to recreational water in the United States in 2014, including 42,300 hospitalizations and 1,290 deaths (CDC, Emerging Infectious Diseases, 2023). That model covers lakes, rivers and oceans, sporadic illness and swimmer’s ear (79% of the estimate), so it is not directly comparable with outbreak counts; it shows that outbreak reports capture a small share of swimming-related illness. Across all water exposures, CDC estimates that 1 in 44 people in the United States gets sick from waterborne disease each year (CDC).

For comparison across water types, CDC counted 214 drinking water outbreaks reported from 2015 to 2020, most of them caused by Legionella (CDC MMWR, 2024).

Chlorine, hot tub water levels and the Model Aquatic Health Code

CDC publishes recommended disinfectant levels and a model code for public pools, hot tubs and splash pads. The code is guidance, not federal law, and applies only where a state or local jurisdiction adopts it.

• Hot tubs: free chlorine at least 3 ppm, or bromine 4 to 8 ppm, with pH 7.0 to 7.8 and water no hotter than 104 F (CDC Healthy Swimming).
• Diarrheal contamination response: MAHC hyperchlorination raises free chlorine to 20 ppm for 12.75 hours to inactivate Cryptosporidium (CDC MMWR, 2021).
• 2024 MAHC, 5th edition (posted December 16, 2024): cyanuric acid at 300 ppm or higher requires immediate remediation, and free chlorine above 10 ppm with bathers present is an immediate-closure hazard (CDC MAHC).
• CDC states that one in eight routine public pool inspections and one in seven routine public hot tub or spa inspections result in immediate closure (CDC MAHC).
• CDC found improper disinfectant levels in about 20% of 13,864 routine public hot tub inspections in 16 jurisdictions in 2013 (CDC MMWR, 2018).

What the record shows

1. In 2023, the latest onset year in CDC’s public file, U.S. health departments reported 50 treated recreational water outbreaks with 379 cases, 60 hospitalizations and 3 deaths (CDC NORS).
2. The 2023 total of 50 outbreaks is the highest annual count in the U.S. file since 2018, when 56 were reported (CDC NORS).
3. Legionella was the cause of 28 of the 50 U.S. outbreaks with onset in 2023, and hot tubs or spas alone were the venue in 26 (CDC NORS).
4. Only 9 U.S. treated recreational water outbreaks had onset in 2020, 80.7% below the 2015 to 2019 annual mean, during the first year of the COVID-19 pandemic (PC&S analysis of CDC NORS).
5. Legionella outbreaks outnumbered Cryptosporidium outbreaks in every onset year from 2017 to 2023 in the United States (PC&S analysis of CDC NORS).
6. Cryptosporidium outbreaks fell from 91 in 2015 to 2019 to 10 in 2020 to 2023 in the U.S. file, an 86.3% drop in the annual rate (PC&S analysis of CDC NORS).
7. The two largest U.S. outbreaks from 2009 to 2023 were Cryptosporidium hominis outbreaks with onset in July 2016: 638 cases in Ohio and 459 cases in Arizona (CDC NORS).
8. Eleven U.S. Cryptosporidium outbreaks from 2009 to 2023 each sickened more than 100 people, together accounting for 2,294 cases (CDC NORS).
9. The median U.S. treated recreational water outbreak from 2009 to 2023 had 5 cases; the median Cryptosporidium outbreak had 9 and the median Legionella outbreak had 2 (CDC NORS).
10. Pools alone were the venue in 269 U.S. outbreaks (48.0%) and 4,862 cases (56.9%) from 2009 to 2023 (PC&S analysis of CDC NORS).
11. Hot tub or spa outbreaks produced 33.8 hospitalizations per 100 reported cases in the United States from 2009 to 2023, against 3.5 per 100 for pool outbreaks (PC&S analysis of CDC NORS).
12. Chemical exposures (chlorine, chlorine gas, chloramines and acids) caused 68 U.S. outbreaks and 1,293 cases, 15.1% of cases, from 2009 to 2023 (PC&S analysis of CDC NORS).
13. Hotel, motel or resort settings accounted for 199 of 560 U.S. outbreaks (35.5%) from 2009 to 2023 (PC&S analysis of CDC NORS).
14. Florida (79), Minnesota (64) and Ohio (51) reported the most U.S. treated recreational water outbreaks from 2009 to 2023 (CDC NORS).
15. CDC estimates 5.61 million illnesses in the United States in 2014 were linked to all recreational water, about 9,590 for every case counted in that year’s treated-water outbreaks (CDC, Emerging Infectious Diseases, 2023; PC&S calculation).

Patterns across the registry

• CDC’s public outbreak file lists 560 treated recreational water outbreaks with first illness from 2009 to 2023 in the United States, an average of 37.3 a year (PC&S analysis of CDC NORS).
• Those 560 outbreaks caused 8,539 cases of illness, 689 hospitalizations and 22 deaths in the United States, 2009 to 2023 (CDC NORS).
• Cryptosporidium, a chlorine-tolerant parasite, caused 57.9% of U.S. outbreak cases from 2009 to 2023, but only 30.5% from 2019 to 2023 (PC&S analysis of CDC NORS).
• Legionella caused all 22 outbreak deaths and 59.9% of outbreak hospitalizations in the United States, 2009 to 2023 (PC&S analysis of CDC NORS).
• Hot tubs or spas alone were the venue in 176 U.S. outbreaks from 2009 to 2023; Legionella or Pseudomonas caused 93.8% of them (PC&S analysis of CDC NORS).
• 49.5% of U.S. treated recreational water outbreaks from 2009 to 2023 began in June, July or August; 83.2% of Cryptosporidium outbreaks did (PC&S analysis of CDC NORS).
• Reported counts keep rising after first publication: the 2021 count grew from 22 outbreaks in CDC’s 2021 summary to 30 in the current file (CDC WBDOSS 2021 summary; CDC NORS).

Counts and rates we derived

Download the calculation sheet (CSV)

Recent entries

• 2026-07CDC’s July 2, 2026 MMWR described two legionellosis outbreaks at U.S. Virgin Islands hotels in November 2024 traced to shower and sink water, not pools or hot tubs; they are outside this registry’s scope (CDC MMWR, 2026).
• 2026-06CDC’s June 11, 2026 MMWR linked 2 Legionnaires disease cases in October 2024 to a private hot tub at a New York vacation rental, using whole genome sequencing (CDC MMWR, 2026).
• 2025-03CDC’s March 27, 2025 MMWR described a fatal Naegleria fowleri infection in a child aged 16 months linked to an inadequately disinfected splash pad in Arkansas in September 2023, the third splash pad associated case reported in the United States (CDC MMWR, 2025).
• 2024-12CDC’s public NORS file was last updated on December 20, 2024 and runs through onset year 2023; as of October 5, 2026 no 2024 onset-year update had been posted (CDC NORS).
• 2024-12CDC posted the 5th edition of the Model Aquatic Health Code on December 16, 2024, adding immediate remediation at cyanuric acid of 300 ppm or higher (CDC MAHC).
• 2024-12CDC’s December 2024 surveillance summary counted 60 splash pad outbreaks and 10,611 cases from 1997 to 2022 (CDC MMWR, December 2024).

Questions about the registry

Other questions

Source types

Tier 1: primary and government sources (26)

• Centers for Disease Control and Prevention (CDC): National Outbreak Reporting System (NORS) public dataset, data.cdc.gov 5xkq-dg7x
• CDC: NORS Data: accessing, using and analyzing NORS data
• CDC, Morbidity and Mortality Weekly Report: Outbreaks Associated with Treated Recreational Water, United States, 2015 to 2019 (MMWR 70(20):733 to 738, May 21, 2021)
• CDC, Morbidity and Mortality Weekly Report: Outbreaks Associated with Treated Recreational Water, United States, 2000 to 2014 (MMWR 67(19):547 to 551, 2018)
• CDC: Waterborne Disease and Outbreak Surveillance System (WBDOSS) Summary Report, United States, 2021
• CDC, Morbidity and Mortality Weekly Report: Waterborne Disease Outbreaks Associated with Splash Pads, United States, 1997 to 2022 (MMWR Surveillance Summaries 73(8), December 5, 2024)
• CDC, Morbidity and Mortality Weekly Report: Surveillance of Waterborne Disease Outbreaks Associated with Drinking Water, United States, 2015 to 2020 (MMWR Surveillance Summaries 73(1), March 14, 2024)
• CDC, Morbidity and Mortality Weekly Report: Pseudomonas Infection Outbreak Associated with a Hotel Swimming Pool, Maine, March 2023 (MMWR 73(2):32 to 36, January 18, 2024)
• CDC, Morbidity and Mortality Weekly Report: Fatal Case of Splash Pad Associated Naegleria fowleri Meningoencephalitis, Pulaski County, Arkansas, September 2023 (MMWR 74(10):167 to 172, March 27, 2025)
• CDC, Morbidity and Mortality Weekly Report: Legionnaires Disease Associated with a Private-Use Hot Tub in a Vacation Rental Property, New York, October 2024 to April 2025 (MMWR 75(22):274 to 279, June 11, 2026)
• CDC, Morbidity and Mortality Weekly Report: Outbreaks of Acute Gastrointestinal Illness Associated with a Splash Pad in a Wildlife Park, Kansas, June 2021 (MMWR 71(31):981 to 987, August 5, 2022)
• CDC Healthy Swimming: Swimming and Your Health
• CDC Healthy Swimming: Preventing Swimming-related Illnesses
• CDC Healthy Swimming: Preventing Diarrheal Illnesses
• CDC: About Crypto Infections
• CDC: About Legionnaires' Disease
• CDC Healthy Swimming: About Protecting Yourself from Legionella in Hot Tubs
• CDC Healthy Swimming: Preventing Hot Tub Rash
• CDC Healthy Swimming: What You Can Do to Stay Healthy in Hot Tubs
• CDC: About The Model Aquatic Health Code
• CDC: About The MAHC Current Edition (2024 MAHC, 5th edition)
• CDC, Emerging Infectious Diseases: Estimating Waterborne Infectious Disease Burden by Exposure Route, United States, 2014 (Gerdes et al., Emerging Infectious Diseases 29(7), July 2023)
• CDC: Estimate of Waterborne Disease Burden in the United States
• Minnesota Department of Health: Recreational Water Illnesses (RWIs)
• Norwegian Institute of Public Health authors; Elsevier (abstract via PubMed): Swimming pool-associated infectious disease outbreaks from 2020 to 2025: an updated literature review (Steinberg et al., Int J Hyg Environ Health 274:114787, 2026)
• CDC, Morbidity and Mortality Weekly Report: Legionellosis Outbreaks Associated with Two Hotels, U.S. Virgin Islands, October 2024 to April 2025 (MMWR 75(25):322 to 326, July 2, 2026)

Primary records

1. Centers for Disease Control and Prevention (CDC). National Outbreak Reporting System (NORS) public dataset, data.cdc.gov 5xkq-dg7x. Accessed 2026-10-05.
2. CDC. NORS Data: accessing, using and analyzing NORS data. Accessed 2026-10-05.
3. CDC, Morbidity and Mortality Weekly Report. Outbreaks Associated with Treated Recreational Water, United States, 2015 to 2019 (MMWR 70(20):733 to 738, May 21, 2021). Accessed 2026-10-05.
4. CDC, Morbidity and Mortality Weekly Report. Outbreaks Associated with Treated Recreational Water, United States, 2000 to 2014 (MMWR 67(19):547 to 551, 2018). Accessed 2026-10-05.
5. CDC. Waterborne Disease and Outbreak Surveillance System (WBDOSS) Summary Report, United States, 2021. Accessed 2026-10-05.
6. CDC, Morbidity and Mortality Weekly Report. Waterborne Disease Outbreaks Associated with Splash Pads, United States, 1997 to 2022 (MMWR Surveillance Summaries 73(8), December 5, 2024). Accessed 2026-10-05.
7. CDC, Morbidity and Mortality Weekly Report. Surveillance of Waterborne Disease Outbreaks Associated with Drinking Water, United States, 2015 to 2020 (MMWR Surveillance Summaries 73(1), March 14, 2024). Accessed 2026-10-05.
8. CDC, Morbidity and Mortality Weekly Report. Pseudomonas Infection Outbreak Associated with a Hotel Swimming Pool, Maine, March 2023 (MMWR 73(2):32 to 36, January 18, 2024). Accessed 2026-10-05.
9. CDC, Morbidity and Mortality Weekly Report. Fatal Case of Splash Pad Associated Naegleria fowleri Meningoencephalitis, Pulaski County, Arkansas, September 2023 (MMWR 74(10):167 to 172, March 27, 2025). Accessed 2026-10-05.
10. CDC, Morbidity and Mortality Weekly Report. Legionnaires Disease Associated with a Private-Use Hot Tub in a Vacation Rental Property, New York, October 2024 to April 2025 (MMWR 75(22):274 to 279, June 11, 2026). Accessed 2026-10-05.
11. CDC, Morbidity and Mortality Weekly Report. Outbreaks of Acute Gastrointestinal Illness Associated with a Splash Pad in a Wildlife Park, Kansas, June 2021 (MMWR 71(31):981 to 987, August 5, 2022). Accessed 2026-10-05.
12. CDC Healthy Swimming. Swimming and Your Health. Accessed 2026-10-05.
13. CDC Healthy Swimming. Preventing Swimming-related Illnesses. Accessed 2026-10-05.
14. CDC Healthy Swimming. Preventing Diarrheal Illnesses. Accessed 2026-10-05.
15. CDC. About Crypto Infections. Accessed 2026-10-05.
16. CDC. About Legionnaires' Disease. Accessed 2026-10-05.
17. CDC Healthy Swimming. About Protecting Yourself from Legionella in Hot Tubs. Accessed 2026-10-05.
18. CDC Healthy Swimming. Preventing Hot Tub Rash. Accessed 2026-10-05.
19. CDC Healthy Swimming. What You Can Do to Stay Healthy in Hot Tubs. Accessed 2026-10-05.
20. CDC. About The Model Aquatic Health Code. Accessed 2026-10-05.
21. CDC. About The MAHC Current Edition (2024 MAHC, 5th edition). Accessed 2026-10-05.
22. CDC, Emerging Infectious Diseases. Estimating Waterborne Infectious Disease Burden by Exposure Route, United States, 2014 (Gerdes et al., Emerging Infectious Diseases 29(7), July 2023). Accessed 2026-10-05.
23. CDC. Estimate of Waterborne Disease Burden in the United States. Accessed 2026-10-05.
24. Minnesota Department of Health. Recreational Water Illnesses (RWIs). Accessed 2026-10-05.
25. Norwegian Institute of Public Health authors; Elsevier (abstract via PubMed). Swimming pool-associated infectious disease outbreaks from 2020 to 2025: an updated literature review (Steinberg et al., Int J Hyg Environ Health 274:114787, 2026). Accessed 2026-10-05.
26. CDC, Morbidity and Mortality Weekly Report. Legionellosis Outbreaks Associated with Two Hotels, U.S. Virgin Islands, October 2024 to April 2025 (MMWR 75(25):322 to 326, July 2, 2026). Accessed 2026-10-05.