A compiled reference of 21 verified statistics on Legionnaires' disease and the building water systems that spread it: the two-decade rise in reported cases, how severe and costly the illness is, where outbreaks actually originate, the water-management failures behind them, and Legionella's dominance among US drinking-water outbreaks. Every figure is drawn from a named public source (CDC surveillance, Vital Signs, and economic-burden data, plus the ANSI/ASHRAE Standard 188 water-management framework) and independently checked before publishing.
Where outbreaks come from
Sources of Legionnaires' disease outbreaks investigated in North America, 2000-2014 (27 outbreaks).
Source: CDC Vital Signs, Legionnaires' disease outbreaks, North America 2000-2014 (cdc.gov). Building potable water is the single largest source.
Key statistics
Key takeaways
Legionnaires' disease is a building-systems problem as much as a medical one: it grows in the water we pipe, aerosolize, and cool, and CDC investigations find a management deficiency behind essentially every outbreak. The numbers below trace the rise, the toll, and the source.
Reported Legionnaires' disease cases rose about nine times since 2000, with nearly 10,000 reported in 2018.
About 1 in 10 people who get Legionnaires' disease die; about 1 in 4 die if it is contracted in a healthcare facility.
56% of investigated outbreaks (2000-2014) traced to building potable water; 22% to cooling towers.
Every outbreak with enough information to evaluate had at least one water-management deficiency.
Legionella caused 97% of hospitalizations and 98% of deaths among US drinking-water outbreaks (2015-2020).
The 2014 US burden was an estimated 11,000 cases, 995 deaths, and $835 million in cost of illness.
How we built this report
Every figure was compiled in September 2026 from named public sources and verified against the original before publishing.
- Compiled from primary sources
CDC surveillance summaries, CDC Vital Signs, and CDC economic-burden and incidence studies, plus the ANSI/ASHRAE Standard 188 water-management framework.
- Government data for every health claim
Case, death, source, and outbreak figures come only from the CDC, read from the original surveillance publications rather than secondary summaries.
- Disagreements shown, not averaged
Where reported cases and estimated true burden differ, we show both and label which is which.
- Independent review
Written by one co-founder, reviewed by the other before publishing.
Scope note: reported case counts are surveillance figures the CDC considers underestimates because Legionnaires' disease is underdiagnosed, so the burden estimates (11,000 cases, 995 deaths) run higher than reported counts. Outbreak-source percentages are from the subset of outbreaks with investigation data, not all cases; roughly 64% of reported cases have no identified source. ASHRAE Standard 188 is a consensus standard, adopted into requirements by some jurisdictions and healthcare programs but not universally mandated.
Legionnaires' disease, by the numbers
All 21 figures, grouped into four themes, each from a named public source and independently verified against the original.
A two-decade rise
Legionnaires' disease has climbed steadily since 2000, and the CDC attributes the rise partly to aging building water systems. According to the CDC (cdc.gov), reported cases grew about nine times since 2000, with nearly 10,000 reported in 2018, and incidence research shows the crude rate rising from 0.55 to 2.71 cases per 100,000 people between 1992 and 2018.
Statistic 1
Reported Legionnaires' disease cases rose about nine times since 2000, with nearly 10,000 reported in 2018 (considered an underestimate).
CDC (2026)
Statistic 2
An average of 1,221 cases per year were reported in 1992-2002, rising to an average of 4,369 per year in 2003-2018.
CDC Emerging Infectious Diseases (2022)
Statistic 3
Crude incidence rose from 0.55 per 100,000 (1992) to 2.71 per 100,000 (2018), a 5.67x risk ratio.
CDC Emerging Infectious Diseases (2022)
Statistic 4
From 2002 to 2018, age-standardized incidence rose an average of 9.3% per year.
CDC Emerging Infectious Diseases (2022)
Statistic 5
During 2000-2014, the rate of reported legionellosis increased 286%, from 0.42 to 1.62 cases per 100,000.
CDC (2016)
What this means: This is not a diagnostic artifact alone. The CDC ties the climb to aging plumbing and water infrastructure and an aging, more susceptible population, which puts the building operator, not just the clinician, on the front line.
How severe, and what it costs
Legionnaires' disease is a severe pneumonia, not a mild waterborne bug. According to the CDC (cdc.gov), roughly 95% of patients require hospitalization and about 1 in 10 die, rising to 1 in 4 among those who contract it in a healthcare facility. A CDC burden study put the 2014 US cost of illness at $835 million.
Statistic 6
About 95% of Legionnaires' disease patients require hospitalization.
CDC Emerging Infectious Diseases (2022)
Statistic 7
About 1 in 10 people who get Legionnaires' disease die from complications.
CDC (2026)
Statistic 8
About 1 in 4 die if they contract Legionnaires' disease during a stay in a healthcare facility.
CDC (2026)
Statistic 9
About 64% of reported cases have no known exposure and lack an identified source.
CDC Emerging Infectious Diseases (2022)
Statistic 10
The 2014 US burden was an estimated 11,000 domestically acquired cases and 995 deaths, with a total cost of illness of $835 million.
CDC Emerging Infectious Diseases (2021)
Statistic 11
Of that $835 million, about $402 million was medical cost and about $412 million was lost productivity from premature deaths.
CDC Emerging Infectious Diseases (2021)
What this means: The high hospitalization and fatality rates are why a building's water system is a genuine liability surface, not just a maintenance line item, especially for operators serving older or medically vulnerable guests and residents.
Building water systems as the source
The disease originates in the buildings we operate. According to CDC Vital Signs (cdc.gov), in outbreaks investigated from 2000 to 2014, 56% were associated with building potable water, 22% with cooling towers, and 7% with hot tubs, and among the outbreaks with enough information to evaluate, every single one had at least one water-management deficiency. The ANSI/ASHRAE Standard 188 framework exists to close exactly those gaps.
Statistic 12
Of 27 investigated outbreaks (2000-2014), 56% traced to building potable water, 22% to cooling towers, and 7% to hot tubs.
CDC Vital Signs (2016)
Statistic 13
Of the 23 outbreaks with evaluable information, all had at least one water-management deficiency; 48% had deficiencies in two or more categories.
CDC Vital Signs (2016)
Statistic 14
Outbreak deficiencies broke down as process failures (65%), human errors (52%), equipment failures (35%), and unmanaged external changes (35%).
CDC Vital Signs (2016)
Statistic 15
Higher-risk groups include adults 50 and older, current or former smokers, and people with chronic lung disease or weakened immune systems.
CDC (2026)
Statistic 16
ANSI/ASHRAE Standard 188 sets water-management program requirements for commercial, institutional, multiunit residential, and industrial buildings, excluding single-family homes.
ANSI/ASHRAE Standard 188 (2021)
Statistic 17
Standard 188 requires a water-management plan for buildings with cooling towers, spas, or fountains, extending to potable water where legionellosis risk factors are present.
ANSI/ASHRAE Standard 188 (2021)
What this means: That every evaluated outbreak had a management deficiency is the single most actionable fact here: these are operational failures, not acts of nature. The prevention is a documented water-management program, the same discipline of surveying, recording, and verifying condition that governs the rest of a building's systems.
The leading waterborne threat
Among US drinking-water outbreaks, Legionella now dominates the serious end. According to CDC surveillance (cdc.gov), of 214 drinking-water-associated outbreaks reported from 2015 to 2020, Legionella was the leading cause, responsible for 97% of hospitalizations and 98% of deaths, and it accounted for 98% of all biofilm-associated outbreaks.
Statistic 18
Of 214 drinking-water-associated outbreaks reported in 2015-2020, Legionella was the leading cause.
CDC MMWR Surveillance Summary (2024)
Statistic 19
Legionella caused 786 (37%) of illnesses, 544 (97%) of hospitalizations, and 86 (98%) of deaths in those outbreaks.
CDC MMWR Surveillance Summary (2024)
Statistic 20
Legionella accounted for 98% of biofilm-associated drinking-water outbreaks (184 of 187).
CDC MMWR Surveillance Summary (2024)
Statistic 21
Reported Legionella drinking-water outbreaks rose from 14 in 2015 to a peak of 34 in 2018, most linked to public water systems (160 of 184).
CDC MMWR Surveillance Summary (2024)
What this means: Legionella has become the dominant cause of serious waterborne illness in the US precisely because it lives in the built environment's plumbing, downstream of the treatment plant, where only the building's own management program can control it.
Cite this study
Academic or press use: copy a ready-made reference. RapidEye is the publisher.
Quick FAQ
How many Legionnaires' disease cases are there per year?
Nearly 10,000 cases of Legionnaires' disease were reported in the US in 2018, and reported cases have risen about nine times since 2000, according to the CDC (cdc.gov). The CDC considers this an underestimate because the disease is underdiagnosed; a burden study estimated about 11,000 domestically acquired cases in 2014.
How deadly is Legionnaires' disease?
About 1 in 10 people who get Legionnaires' disease die from complications, and about 1 in 4 die if they contract it during a stay in a healthcare facility, according to the CDC (cdc.gov). Roughly 95% of patients require hospitalization.
Where does Legionella come from in buildings?
In a CDC review of outbreaks from 2000 to 2014, 56% were associated with building potable water, 22% with cooling towers, and 7% with hot tubs (cdc.gov). Every outbreak with enough information to evaluate had at least one deficiency in water-system maintenance or management, which is why a documented water-management program is the core prevention.
Is Legionella the leading cause of waterborne outbreaks?
Yes. Among drinking-water-associated outbreaks reported in the US from 2015 to 2020, Legionella was the leading cause, responsible for 786 (37%) of illnesses, 544 (97%) of hospitalizations, and 86 (98%) of deaths, according to the CDC (cdc.gov).
Data sources
Every figure traces to one of these named public sources, each checked against the original before publishing. We name each source and show its root domain, but do not hyperlink external sources; internal links stay live.
CDC Vital SignsLegionnaires' disease outbreaks and water-management deficiencies, North America 2000-2014cdc.govCDC Emerging Infectious DiseasesRising incidence and economic-burden studies of Legionnaires' diseasecdc.govCDC MMWR Surveillance SummaryDrinking-water-associated outbreak surveillance, 2015-2020cdc.gov
ANSI/ASHRAE Standard 188Legionellosis risk management for building water systemsashrae.org