Updated September 2026
Clear water isn't the same thing as safe water. Every year, US public health agencies confirm dozens of outbreaks tied to drinking water systems, and the pathogens responsible aren't always the ones people expect. The most current CDC surveillance data shows the leading cause isn't contamination from a dirty source at all, it's bacteria growing inside the water system itself, well after treatment.
Quick answer: The CDC's most recent surveillance report (covering 2015 to 2020, published 2024) recorded 214 drinking water outbreaks across 28 states, resulting in 2,140 illnesses, 563 hospitalizations, and 88 deaths. Legionella, a bacteria that grows inside plumbing and water systems rather than entering from an outside source, caused 98% of the biofilm-related outbreaks and nearly all of the hospitalizations and deaths. Aging infrastructure and premise plumbing, not source-water contamination, are now the dominant drivers of US waterborne illness.
How Often Does Contaminated Drinking Water Make People Sick?
The CDC's Waterborne Disease and Outbreak Surveillance System is the most authoritative source for this data, and its latest published report covers outbreaks from 2015 through 2020. Over that period, 214 outbreaks were reported across 28 states, causing 2,140 illnesses, 563 hospitalizations, and 88 deaths. That's a sharp rise from the CDC's prior surveillance window of 2013 to 2014, which recorded 42 outbreaks in 19 states, 124 hospitalizations, and 13 deaths, driven partly by improved detection and partly by a genuine increase in Legionella-associated outbreaks, which the report notes rose from 14 in 2015 to 18 in 2020.
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| Metric (2015-2020) |
Figure |
| Total outbreaks |
214, across 28 states |
| Illnesses |
2,140 |
| Hospitalizations |
563 (26% of cases) |
| Deaths |
88 (4% of cases) |
| Share caused by Legionella |
98% of biofilm outbreaks; 97% of hospitalizations; 98% of deaths |
| Outbreaks from public water systems |
172 (80%) |
Legionella: The Outbreak Cause That Grows Inside the Pipes
Unlike most classic waterborne pathogens, Legionella doesn't typically enter a water system from contaminated source water. It's a naturally occurring bacteria that grows inside plumbing, water heaters, cooling towers, and other building water systems, particularly where water sits warm and stagnant or where chlorine levels have dropped by the time water reaches the tap. That's why the CDC report found more than half of all outbreaks (53%) traced back to healthcare facilities, where complex plumbing systems and vulnerable patient populations combine to create higher risk, with Legionella responsible for 98% of those healthcare-associated cases.
Legionella causes Legionnaires' disease, a severe form of pneumonia, and the milder legionellosis (Pontiac fever), which brings flu-like symptoms such as fever, muscle aches, headache, and difficulty breathing. It's why Legionella accounted for such a disproportionate share of hospitalizations and deaths in the CDC data relative to its share of total illness cases, an infection is far more likely to require hospital care than the enteric illnesses that made up most pre-2015 outbreak reports.
Worth knowing: The CDC report also found that private and individual water systems, which accounted for only 8% of outbreaks, were responsible for a disproportionate 44% of illness cases and 16% of deaths. Private wells aren't regulated or routinely tested the way public utilities are, which is part of why the CDC and EPA both recommend private well owners test their water annually.
Where the Rest of These Outbreaks Come From
The remaining outbreaks in the CDC's dataset stem from a mix of enteric pathogens, including organisms like Cryptosporidium and Giardia that cause gastrointestinal illness, along with a small number of chemical and undetermined causes. These are more likely to be tied to lapses in treatment or distribution, such as inadequate disinfection, cross-connections, or contamination entering through damaged infrastructure.
Aging infrastructure is a real and ongoing factor. According to the EPA, the US experiences an estimated 23,000 to 75,000 sanitary sewer overflows every year, not counting backups into individual buildings, caused by blockages, line breaks, deteriorating pipe joints, and power failures at pump stations. These overflows can contaminate nearby surface water and, in some cases, threaten drinking water sources, which is one reason water infrastructure investment remains a persistent public health issue rather than a one-time fix.
The Real Cost of Waterborne Illness
Outbreak counts only capture the cases large enough to be identified and reported. A 2021 CDC-funded study published in Emerging Infectious Diseases estimated that infectious waterborne disease, including cases never linked to a formal outbreak, causes roughly 7.1 million illnesses in the US every year across 17 tracked pathogens, with an estimated $3.3 billion in direct healthcare costs. That figure covers doctor visits, hospitalizations, and treatment, and doesn't include lost wages or long-term health effects for people who develop chronic complications after a severe infection.
Frequently Asked Questions
What is the leading cause of US drinking water outbreaks today?
Legionella, according to the CDC's most recent surveillance report (2015-2020). It caused 98% of biofilm-related outbreaks and nearly all associated hospitalizations and deaths. Unlike source-water contamination, Legionella typically grows inside building plumbing, water heaters, and other water systems after treatment.
Is public water or private well water more likely to cause an outbreak?
Public water systems account for the majority of reported outbreaks (80%), largely reflecting how many more people are served by public systems. But private and individual water systems, despite only 8% of outbreaks, accounted for 44% of illness cases and 16% of deaths, a disproportionate share that the CDC ties to the lack of routine regulation and testing on private wells.
How many people get sick from waterborne illness in the US each year?
Confirmed outbreaks are only part of the picture. A CDC-funded study estimated roughly 7.1 million total waterborne illness cases occur annually across 17 tracked pathogens, with about $3.3 billion in direct healthcare costs, far more than the cases formally identified as part of a reported outbreak.
Why do so many outbreaks happen in healthcare facilities?
The CDC found 53% of 2015-2020 outbreaks were linked to healthcare facilities, with Legionella responsible for 98% of those cases. Complex plumbing systems, larger building footprints, and higher-risk patient populations combine to make healthcare settings especially vulnerable to Legionella growth and exposure.
Do sewer overflows affect drinking water safety?
They can. The EPA estimates 23,000 to 75,000 sanitary sewer overflows occur in the US annually, caused by aging pipes, blockages, and infrastructure failures. While most overflows affect surface water rather than treated drinking water directly, they're part of the broader infrastructure strain that public health officials point to as an ongoing risk factor.
If my water tests clean, could it still make me sick?
Yes, especially with Legionella, since it grows inside plumbing after water leaves the treatment plant and would not show up in a source-water test. Regular water testing, proper water heater maintenance, and flushing plumbing that's sat unused are all part of standard CDC guidance for reducing that risk.
Concerned About What's in Your Water?
See the full independently lab-tested contaminant list for Black Berkey and Phoenix Elements.
Dan DeBaun
Dan has spent 18 years in the water filtration industry and holds an engineering degree and an MBA from Rutgers University. He owns and operates BigBerkeyWaterFilters.com, the leading US Berkey dealer.
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Dan DeBaun
