Legionella Contamination & Outbreaks

A Worldwide Reference of Incidents, Cases and Fatalities

Key facts

• Legionnaires’ disease is a severe pneumonia caused by Legionella bacteria, generally fatal in about 10% of cases — rising to ~25–50% in hospital-acquired infections when treatment is delayed.

• It is not spread person-to-person; infection occurs by inhaling contaminated water aerosols.

• Bacteria thrive in warm (25–45 °C), stagnant, poorly-maintained water: cooling towers, building plumbing, hot tubs, fountains and humidifiers.

• In the EU/EEA alone, 2021 saw over 10,700 cases and 704 fatalities; the 2024 notification rate reached 3.4 per 100,000.

1. Cooling Towers (Industrial & Community)

Cooling tower systems have caused many of the largest community outbreaks, dispersing contaminated aerosols over entire city blocks. The 2025 Central Harlem cluster is among the most recent major events.

Location / Year (source type)	Cases	Deaths	Reference
Central Harlem, New York City Jul–Aug 2025 · cooling towers	118	7	NEJM Evidence
Melbourne (Laverton North), Australia Aug 2024 · cooling tower	114	2	The Lancet Microbe
Lombardy, Italy Aug 2024 · building water	53	4	The Lancet Microbe
London, UK 2024–2025 · cooling tower	30 / 70	2 / 3	Lancet / Wikipedia
Simcoe-Muskoka, Ontario, Canada Sep–Oct 2022 · cooling tower	35	1	Canada CCDR
Bronx, New York City 2015 · cooling tower	138	16	The Lancet Microbe
Murcia, Spain 2001 · cooling tower (largest recorded)	449	—	The Lancet Microbe

Spain 2001 case count is the largest cooling-tower outbreak on record; the Bronx 2015 death figure is as later reported. See CIDRAP outbreak alert and NYC Health update.

2. Hospitals & Healthcare Facilities (Sanitary Water)

Hospital-acquired (nosocomial) cases are the deadliest because patients are older or immunocompromised. The principal source is the building’s potable-water distribution system — sinks, showers, humidifiers and decorative fountains.

Location / Year (source type)	Cases	Deaths	Reference
Christ Hospital, Cincinnati, Ohio Oct–Dec 2025 · potable water	6	3	Fox19 / Enquirer
UW Hospital, Wisconsin, USA water system	14	3	UW Clipsheet
San Miguel de Tucumán, Argentina Aug 2022 · private health facility	22	6	Wikipedia
Pas-de-Calais, France 2003–2004 · industrial (petrochem.)	86	18	Wikipedia (list)
USA — building plumbing (CDC MMWR) 2011–2012 · potable systems	430+	14	CBS News / CDC

Background on healthcare water risks: The Role of Water in Healthcare-Associated Infections (PMC).

3. Hot Tubs, Jacuzzis & Spas

Hot tubs operate at 30–40 °C — ideal for Legionella — while heat rapidly breaks down chlorine. Aeration bursts release fine, bacteria-laden droplets. Even walking past a contaminated display tub has caused infection.

Location / Year (source type)	Cases	Deaths	Reference
N. Carolina Mountain State Fair Sep 2019 · hot tub display (largest hot-tub outbreak)	136	4	PubMed
Zen Day Spa, Richmond, California 2023 · jacuzzi (unpermitted)	4+	2	Richmond Confidential
Hampton Beach, New Hampshire 2018 · hotel hot tub spas	14	1	Patch
Western New York (rental home) 2024 · private hot tub	2	—	CIDRAP

Aggregate CDC data: 65 recreational-water outbreaks (pools, spas, hot tubs) with 13 deaths were recorded in the US between 2015–2019; Legionella caused 42% of them. MMWR-based summary.

4. Benelux Focus: Belgium, the Netherlands & Luxembourg

All three countries report to the European surveillance network ELDSNet (coordinated by ECDC). The Netherlands has one of Europe’s highest notification rates and pioneered a National Legionella Outbreak Detection Program after the 1999 disaster below.

Location / Year (source type)	Cases	Deaths	Reference
Bovenkarspel, Netherlands Feb 1999 · whirlpool spa, Westfriese Flora flower show (“Legionellaramp”)	206	32	Wikipedia
Evergem, Belgium May 2019 · industrial cooling towers along canal	30	2	Legionella Control
Dendermonde, Belgium Aug–Sep 2016 · cooling tower (10 confirmed + 7 probable/possible)	17	—	PubMed
Steinfort, Luxembourg Jul 2025 · Hôpital Intercommunal de Steinfort (HIS) water system	7–9	0	Chronicle.lu

The 1999 Bovenkarspel outbreak (often dated “1990” in secondary sources) is the deadliest European outbreak on record and second only to Philadelphia 1976 historically. In the Netherlands’ 2002–2012 detection program, garden centres (26%) and hospital/healthcare settings (16%) were the leading identified cluster sources. Dutch NLODP study (CDC EID). The 2025 Steinfort cluster in Luxembourg was linked to a hospital and remains under investigation; the Belgian clusters were traced to industrial cooling towers.

5. Fatalities at a Glance

Reported deaths across a selection of the outbreaks above, colour-coded by exposure source:

Note: case and death counts reflect figures at the time of reporting and may have been revised. Historic hotel/spa events (Philadelphia 1976, Bovenkarspel 1999) remain among the deadliest single outbreaks ever recorded.