Legionnaires’ disease may not be a common bacterial infection, but in its short history of existence, it’s been the cause of many serious illnesses and deaths throughout the world.

With initial symptoms reflecting those of the common influenza virus or forms of pneumonia, a diagnosis for Legionnaires takes more extensive testing, allowing symptoms to worsen and more people to be exposed.

This dangerous strain of bacteria spreads through infected water or aerosols in areas where there’s poor air ventilation.

The worst outbreak in history exposed the bacterium to nearly 16,000 people before it was contained, and in other cases, death rates reached into the double digits.

Take a look at the 12 worst Legionnaires’ outbreaks in the world, listed in chronological order:

1. Philadelphia, PA, USA |  1976
Source: Air conditioning
Death-to-infection rate: 34/221, or 15.4%
Fact: This was the first recorded outbreak of the disease, and the mysterious strain of bacteria that caused it was finally identified. 

 2. Stafford, England | 1985
Source: Hospital air conditioning unit
Death-to-infection rate: 28/175, or 16%
Fact: This marked the largest outbreak of Legionnaires disease in Britain.

3. Bovenkarspel, Netherlands | 1999
Source: Hot tub
Death-to-infection rate: 32/318, or 10%.
Fact: More deaths from this bacteria could have been recorded, but bodies had already been buried before they had identified Legionnaires as the cause of the outbreak.

4. Melbourne, Australia | 2000
Source: Cooling tower at a newly opened aquarium
Death-to-infection rate:  4/95, or 4.2%
Fact: 100% of those diagnosed with the disease had visited or passed within 500 m of the the Melbourne Aquarium in the same 4 week period, allowing scientists to pinpoint the exact source of exposure.

5. Murcia, Spain | 2001
Source: Hospital air conditioning
Death-to-infection rate: 6/100, or 1%
Fact: This is the world’s largest outbreak of the disease, where at least 16,000 people were exposed to the bacterium.

6. Barrow-in-Furness, England | 2002
Source: Contaminated cooling tower in a neighborhood arts center
Death-to-infection rate: 7/172, or 4%
Fact: The Barrow Borough Council and the architect of the arts center were charged with, but cleared of, corporate manslaughter charges, and were later fined for breaches of health and safety regulations.

7. Lens, France | 2004
Source: Cooling tower at a petrochemical plant
Death-to-infection rate: 7/59, or 11.8%
Fact: This marked the worst Legionnaires outbreak ever recorded in France.

8. Zaragoza, Spain | 2004
Source: Hospital cooling tower
Death-to-infection rate: 7/27, or 26%
Fact: Five of the seven victims of this outbreak were above the age of fifty.

9. Fredrikstad, Norway | 2005
Source: Air scrubber of a nearby factory
Death-to-infection rate: 10/56, or 17.8%
Fact: The patients that became ill in this outbreak lived over 20 km apart, but the high velocity, large drift and high humidity in the air scrubber were linked to the long-distance spread.

10. New Brunswick, NJ, USA | 2008
Source: Contaminated supply of drinking water
Death-to-infection rate: 2/6,  or 33%
Fact: Exposure to patients in the hospital where the outbreak occurred has been traced back to a sudden drop in chlorine levels in the hospital’s drinking water.

11. Dayton, OH, USA | 2011
Source: Hospital air conditioning
Death-to-infection rate: 5/11, or 45%
Fact: This was the largest outbreak of Legionnaires’ disease in Ohio since 1994.

12. Calpe, Spain | 2012
Source: Currently unknown
Death-to-infection rate: 3/18, or 17%
Fact: The infection quickly spread through a beach hotel and claimed the lives of three British vacationers before being contained.

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