Risk reduction and Legionella
To prepare for the arrival of hundreds of children returning to school after the long summer, my wife ran the school taps to flush out stagnant water. As a microbiologist, married to a microbiologist (who manages a school) I’m interested to explore incidences of Legionella — waterborne, pathogenic bacteria — in the UK and further afield. I’m also interested in the broader aspects of risk management in daily life, and professionally, in product development projects. I’m intrigued by the adage that risks can be managed and reduced but not removed.
Legionellosis — a brief history
Legionellosis - any disease caused by Legionella — is relatively new, and can be traced back to 1976 when a large group of people attending an American Legion Convention became ill with a mysterious illness with symptoms similar to pneumonia: 29 died from what became known as Legionnaires’ disease. It wasn't until 1977 that the cause was identified as an outbreak of L. pneumophila. Since then, legionellosis outbreaks have been recorded around the world.
What is Legionella?
Legionella is a genus of bacteria that includes several species, the most well-known is L. pneumophila. To date, greater then 60 Legionella species have been described and 30 are considered pathogenic. Transmitted through inhalation of contaminated water droplets, Legionella take 2–10 days to incubate, producing symptoms associated with severe respiratory illness, including high fever, chills, cough and pneumonia.
Discovering Legionella
The discovery of Legionella highlighted the potential dangers of waterborne bacteria in various settings, particularly complex water systems. It can form biofilms - layers of microorganisms that attach to surfaces. Legionella poses a risk to humans when it becomes aerosolised: travelling inside tiny droplets that can be inhaled. Water systems that generate aerosols — cooling towers, showers or air conditioning units — are risk factors for legionellosis. The source at the American Legion Convention was traced to L. pneumophila colonising the air conditioning system in the hotel where the convention took place.
Legionella thrives in warm water environments and has an optimum temperature for growth between 25°C to 50°C. Since its discovery, it has influenced the life of every school, hotel and building with communal water systems, which require regular testing and risk reducing measures (such as running the taps). School water pipes left over a hot summer provide a perfect temperature for Legionella.
Case numbers of legionellosis
Water safety laws have been introduced since the discovery of Legionella to ensure the right precautions to reduce exposure risks are undertaken by businesses. However, as mentioned, risks can be managed but not totally removed: 472 cases of legionellosis were reported in England and Wales in 2024, which is 22% higher than in 2023. Thirteen cases were fatal.
Legionellosis in 2026
In early 2026, an outbreak of Legionnaires’ disease in London was investigated, with 8 confirmed cases. Although detailed investigations were performed, no single source was immediately identified; indicating how complex legionellosis outbreaks can be.
In the UK, guidance and risk management protocols are provided, including how to test for Legionella. This is covered in the new Renters’ Rights Act as landlords must test in a similar way to conducting fire and gas safety inspections.
In the USA, Legionnaires’ disease outbreaks during the recent two summers have resulted in 18 deaths in New York City alone. Consequently, there’s a call to increase cooling tower inspections in the hope of preventing further outbreaks.
This upkeep will become more important as temperatures rise and reliance on air conditioning increases. Risk management measures may result in reduced case numbers but must not lead to complacency.
Author: Simon Walker, Director, RLS. September 2026
#Diagnostics #Legionella #LegonnairesDisease #Legionellosis #RiskManagement #Microbiology
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Further reading and inspiration for this article includes:
A History of Legionnaires’ Disease
Monitoring Legionella in drinking water
Official Statistics Legionellosis in residents of England and Wales: 2024
Legionnaires’ Disease Outbreak in London (2026)
UKHSA Outbreak of Legionnaires’ Disease London
Hampshire landlords warned to guard against deadly legionella and the risk of prison 30th April 2026
New York City to increase cooling tower inspections 30th Aug 2026
Credit: wildpixel
Legionella pneumophila bacteria, which can cause respiratory illnesses known as Legionnaires' disease, a serious form of pneumonia.