The Middlesex-London Health Unit is providing an update to the community following the declaration of an outbreak of Legionnaires’ Disease. To date, 66 cases are attached to the outbreak, including two fatalities. Most of the cases live and/or work in a 6 km radius of one another in the southeast side of the city. The cases ranged in age from 23 to 93.

As part of its commitment to transparency and public safety, the Health Unit developed a map to indicate the general area most impacted by the outbreak. Despite this cluster of cases, the overall risk to the broader community remains low, particularly for individuals who are not at higher risk. Homes and businesses in the area remain open and are expected to follow all recommended health and maintenance protocols. Residents are not discouraged from continuing their daily routines in this area. This map may be adjusted over time, as more information becomes available.

Notes to accompany map:  

• This map shows the general area of confirmed cases of Legionnaires’ disease reported to the Middlesex-London Health Unit in 2025. The blue circle represents a 6-kilometre radius around the average location of all home addresses for reported cases, helping to illustrate where most cases are generally residing.
• This map represents a snapshot in time and should be interpreted in the context of ongoing case and environmental investigations.
• To maintain patient confidentiality, no individual homes or exact addresses of specific cases, or other identifying geographic features, are shown.
• The 6-kilometer radius may not reflect the actual distribution or movement patterns of individuals affected by the outbreak.
• Although this map shows where the greatest density of cases reside, there are confirmed Legionnaires’ cases outside of this area.
• The map is subject to change as more information becomes available as investigations continue. The map reflects the data available at the time of publication.
• The outbreak case definition may change as more information becomes available. Changes in the outbreak definition may increase or decrease the case count and as a result may impact the area shown in the map.

Frequently Asked Questions

What is being done?
The Health Unit is still working in partnership with Public Health Ontario and the Public Health Ontario Laboratories to investigate and identify the source of the outbreak. Environmental sampling of potential sources of Legionella is ongoing.

Is it the same location as last year?
At this time, it is unknown whether it is the same location as last year. However, this is being considered as part of the ongoing investigation.

What should residents and businesses know?

• Local businesses remain safe: Shops, restaurants, and workplaces in the affected area are open and continue implementing standard health protocols. No business closures or restrictions are needed.
• Prevention starts with maintenance: Property owners and operators of water cooling systems are reminded to maintain equipment per regulations. Proper system maintenance can remove the source of the bacteria and help protect the health of our entire community.
• Your role: If you experience symptoms (fever, chills, cough, shortness of breath) within two to ten days of potential exposure, contact your healthcare provider or a walk-in clinic.

Why did it take so long to provide a map of the area?
It took time for our teams to gather sufficient information to map the general area where a localized cluster of Legionnaires’ Disease cases have been reported. While we would have preferred to share this detail when the outbreak was first declared, our priority was to quickly get to work encouraging property owners and operators of water-cooling systems to clean and maintain their systems as a precautionary measure.

Should I wear a mask?
While face masks can decrease your risk of infection with respiratory illnesses, including Legionella, the public risk related to Legionella is low. Therefore, the Health Unit does not recommend regular community-wide face mask use specifically for the prevention of Legionnaire’s disease.

Why are we giving a general area for the outbreak and not a direct location?
Legionella aerosolizes and travels significant distance. The source of the Legionella spread has not yet been found.

Legionella bacteria, which causes Legionnaires’ Disease, occur naturally in soil and water, especially warm water. They grow in environments such as hot tubs, cooling towers, hot water tanks, large plumbing systems, parts of air-conditioning systems in large buildings, and decorative pools or fountains. When systems are not properly cleaned and disinfected, the bacteria can multiply and become aerosolized. People become infected by inhaling this contaminated mist. Tackling this outbreak requires a community-wide effort. Cooling system maintenance plays a key role in removing the source of the bacteria and protecting public health.

For more information about Legionnaires’ Disease: www.healthunit.com/legionnaires-disease

For updated case counts of the Legionnaires’ Disease Outbreak: www.healthunit.com/confirmed-legionnaires-disease-cases. This data will be update weekly on Monday afternoon. The Health Unit will not issue a Public Service Announcement every week.

For the original Outbreak Public Service Announcement, visit: www.healthunit.com/news/middlesex-london-health-unit-declares-legionnaires-disease-o.