There is currently an unusual cluster of cases of illness caused by Legionella bacteria in the Canton of Basel-Stadt. More than 20 people have required or are currently requiring hospital treatment.
Based on current information, the most likely source of infection posing a health risk has been identified and eliminated, while other potential sources are being investigated. The Department of Health is continuing its monitoring and inspection activities until it is certain that the health risk has been fully resolved.
The Medical Services of the Department of Health are currently observing an increased incidence of Legionnaires’ disease. This condition involves pneumonia caused by the bacterium Legionella pneumophila. Infection occurs through the inhalation of atomized water droplets containing Legionella. The disease is not transmitted from person to person.
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Legionella bacteria thrive in water systems where water is not constantly replenished and temperatures range between 25°C and 45°C. Such conditions are found in, among other places, water pipes, taps, showerheads, whirlpools, and the cooling towers of water-based cooling and ventilation systems. Air conditioning units and heat pumps commonly used in private households are not affected.
Current status of cases
As of now, 26 cases of the disease have been confirmed in the Canton of Basel-Stadt. Of these, 25 individuals were hospitalized; nine are currently still receiving inpatient treatment. Seven individuals required treatment in the intensive care unit. Sadly, one person has died, and one person remains in the intensive care unit.
Action taken and measures implemented by the authorities
Epidemiological findings and interviews with the affected patients indicate a cluster of infections in the central Kleinbasel area. Isolated cases have also occurred in other districts. The emergence of the cluster suggests one or more common sources of infection. Consequently, the investigation is focusing primarily on facilities that affect public areas, rather than on private premises.
The investigation has focused on water-operated cooling towers. If such towers are contaminated with Legionella, the water vapor they emit can contain infectious droplets. Depending on weather and wind conditions, these droplets can spread through the air over a wide area. Residents who inhale these droplets risk becoming infected.
In recent days, the Cantonal Laboratory has collected samples from various locations. Of the ten facilities sampled in the vicinity of the cluster, eight exceeded the limit value for Legionella. Two of these facilities showed elevated levels of Legionella pneumophila, the species responsible for the majority of human infections. As a result, and at the behest of the Medical Services, both affected facilities were completely shut down on the morning of Friday, July 31, 2026. Both systems are located on the roof of an office building on Rebgasse and are currently undergoing remediation. In the other six contaminated facilities, only Legionella species that are non-pathogenic to humans have been detected so far. Measures have also been ordered for these facilities to halt any potential Legionella emissions; they currently pose no threat to public health.