What is Legionella?
Legionella pneumophila is a waterborne bacterium which can lead to fatal respiratory diseases in humans, such as Legionnaires’ disease or a milder illness called Pontiac fever.

A worldwide concern
Legionella is naturally occurring and is widespread in ponds, rivers, and other natural water systems. When this bacterium enters water systems in the built environment, conditions such as water temperature and ineffective disinfection levels, can encourage rapid growth.
There are over 60 species of Legionella but Legionella pneumophila is the species that causes over 97% of Legionnaires’ disease outbreaks across the world1. This bacterium also causes a milder illness called Pontiac fever, which triggers flu-like symptoms.
This bacterium has several serogroups. A serogroup is a way to classify bacteria and differentiate its various bacterial strains. Legionella pneumophila serogroup 1 is most often linked with human diseases causing around 80-90%2 of cases, but serogroups 2-15 can also cause pneumonia.
Legionnaires’ disease has a death rate of 10%3 if contracted in the community, rising to 25%4 in a healthcare setting.
Latest known outbreaks
Location | Date | Cases | Deaths |
---|---|---|---|
Illinois, US | 10/2024 | 2 | 0 |
New York, US | 08/2024 - 09/2024 | 24 | 4 |
Missouri, US | 08/2024 | 3 | 0 |
London, UK | 07/2024 - 08/2024 | 25 | 2 |
Melbourne, Aus | 07/2024 - 08/2024 | 109 | 2 |
Lincoln, US | 06/2024 - 07/2024 | 5 | 0 |
Milan, Italy | 04/2024 - 07/2024 | 53 | 4 |
Illinois, US | 04/2024 | 2 | 1 |
What is Legionnaires’ disease?
Legionnaires’ disease is a severe form of pneumonia which can be fatal. Contaminated tiny water droplets (aerosols) can be inhaled where they can infect the lungs. If the infection is not contained or treated early on, it can cause confusion, coma, heart failure and ultimately death. According to the World Health Organisation (WHO) Legionnaires’ disease in untreated immuno-suppressed patients the death rate could be as high as 40-80%.5
Legionnaires’ disease has been seen to cause health related quality of life impacts in 7 out of 8 people6 who have recovered from the disease. They experience one or more long-term symptoms such as:
- Fatigue – extreme tiredness
- Memory Loss – loss of the ability to store and remember information
- Difficulty Concentrating – a decreased ability to focused thoughts
- Cerebellar Dysfunction – balance and coordination difficulties
- Retrograde Amnesia – where memories before contracting the event cannot be recalled
- Post-traumatic Stress Disorder – also known as PTSD, is an anxiety disorder that is a result of a traumatic event.
Why is Legionella pneumophila so dangerous?
Legionella pneumophila serogroup 1 is the primary cause of illness in humans, with serogroups 2-15 also causing illness. Let us unpack how this potentially deadly bacterium has the power to negatively affect human health:
Secretion Systems:
Legionella pneumophila can inject effector proteins into host cells, helping them set up a replicative niche, and sheltering them from our immune defences.
Intracellular Parasitism:
Legionella pneumophilais great at surviving and multiplying inside host cells, especially in human lungs, as it dodges our immune response.
Endotoxin Production:
When this bacterium dies, it releases endotoxins that amplify inflammation in our lungs.
Considering Legionella pneumophila’s virulence and its adaptability it is a pivotal public health concern. So, the best way to avoid it ever reaching human lungs is to find it, fast!
Any water source producing aerosols should be considered at risk for transmission of Legionella bacteria, including cooling towers, spa and pools, fountains, showers, taps, car washes, air conditioning units, misters, hoses, and humidifiers amongst others.
Why is VBNC Legionella a real threat?
VBNC Legionella stands for Viable But Non-Culturable Legionella. This refers to a state where Legionella bacteria are still alive but cannot be detected using standard laboratory culturing methods. In this state:
- Alive, but dormant: The bacteria are alive and capable of causing harm but are in a dormant or stressed state, making them undetectable by traditional lab tests.
- Triggered by stress: VBNC occurs in response to environmental stresses, such as temperature changes, nutrient depletion, or exposure to disinfectants in water systems.
- Health risks remain: Even though they are undetectable by culturing, VBNC Legionella can become active and infectious when conditions improve, posing a serious health risk.
Getting repeat positives?
This could be due to VBNC Legionella hiding in your water system. The Hydrosense tests are designed to detect all viable Legionella, including VBNC forms, by identifying bacterial antigens directly. This rapid test gives you accurate results in as little as 25 minutes, helping you take immediate action to protect health and safety.
Hydrosense helps you see what traditional methods might miss. Are you ready to uncover the hidden risk?
Legionella outbreaks are on the rise worldwide
Since 2000, there has been a significant increase in reported cases of Legionnaires’ disease in both the United States and Europe. In the US, the incidence of Legionnaires’ disease increased from 0.42 cases per 100,000 people in 2000 to 2.71 cases per 100,000 in 20197, representing a more than fivefold increase. Similarly, in Europe, the notification rate reached its highest ever in 2021, with 2.4 cases per 100,000 people8, and countries like Italy, France, Spain, and Germany accounted for 75% of all reported cases.
The COVID-19 pandemic caused a notable drop in Legionnaires’ disease cases due to reduced travel and the shutdown of many facilities. However, with the easing of lockdown restrictions and the resumption of travel, the number of cases increased again. In Europe, there was a 38% increase in travel-associated Legionnaires’ disease cases in 2021 compared to 20207.
Detecting Legionella pneumophila quickly is essential to enable action to be taken to reduce the risk of exposure to anyone using water facilities.
Stay informed of news, events and product updates.
Join our mailing list
Sectors at risk
Wherever water systems are present in the built environment, there is a risk of Legionella. Whilst some sectors and industries pose a higher risk such as healthcare and hospitality, all have the potential to be affected.
Hydrosense tests offer the perfect solution for any industry where reputation is crucial, staff and customer safety are paramount or where facilities are in remote locations. Our tests ensure quick, reliable detection, helping to maintain a safe environment and protect public health in industries such as:
- Cooling towers
- Hospitals & Care Homes
- Hospitality & Recreational
- Facilities Management
- Spa pools & Fountains
- Construction
- Shipping
- Offshore
- Car washes
- Domestic Water Systems
An innovative approach
At Hydrosense, we are dedicated to continuous improvement, ensuring our products meet our promise of delivering rapid and reliable Legionella pneumophila testing. Our experts leverage innovation and technology to provide easy-to-use onsite solutions to detect this dangerous bacterium in real time.
With Hydrosense, you can trust that you’re partnering with a team who is committed to protecting public health. Explore more to see why Hydrosense can help you safeguard your community with confidence and ease.
Latest news

Behind Hydrosense PRO: Making Legionella testing see more
Legionella pneumophila (L. pneumophila) serogroup 1 has long been a major focus here at Hydrosense and we are passionate about the part we play in tackling this harmful water

Should we worry about non-pneumophila Legionella species?
Non-pneumophila Legionella Species – Serious concern or distraction from major risk? The genus Legionella is a genus of the family Legionellaceae. It is composed of

Biofilm assessment is crucial to your Legionella risk assessment
When considering important factors required for a well-maintained water system, the most common elements that come to mind may be disinfection of the water via