The Legionella outbreak in the canton of Basel has grown to 28 confirmed cases, while six people remain in hospital, and prosecutors have opened criminal proceedings following one death linked to the outbreak. Authorities believe two contaminated cooling towers in Kleinbasel were the most likely source of the infections, but the investigation into the exact circumstances is continuing.
Legionnaires’ disease is a form of pneumonia caused by the bacterium Legionella pneumophila. Symptoms can include fever, breathing difficulties and pneumonia-like illness.
Infection occurs through the inhalation of aerosolised water droplets containing Legionella bacteria. The disease is not transmitted from person to person.
Cases Increase From 26 To 28
Basel’s Health Department initially reported 26 cases of Legionnaires’ disease over a two-week period. By Wednesday, two additional infections had been confirmed, bringing the total linked to the outbreak to 28.
One of the newly infected people required hospital treatment, while another patient who had been receiving intensive care was transferred out of the ICU, according to the latest update. Six people are currently still receiving inpatient treatment.
The first official update said that 25 of the original 26 patients had been hospitalised, while seven had required intensive care. One person died after contracting the disease, and another remained in intensive care at the time of the initial announcement.
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Basel Legionella Outbreak: Cooling Towers Identified As Likely Source
The outbreak is concentrated mainly in central Kleinbasel, although isolated cases have also been recorded elsewhere in the city. The geographical pattern led health officials to search for a shared environmental source rather than individual household infections.
Investigators focused on water-based evaporative cooling systems, which can release fine droplets into the air if contaminated with Legionella bacteria. Authorities tested 10 cooling facilities near the affected area. Eight exceeded the applicable limit for Legionella bacteria, but two systems showed elevated levels of Legionella pneumophila, the species responsible for most human cases of Legionnaires’ disease.
The two systems were located on the roof of an office building on Rebgasse in Kleinbasel and were completely shut down on Friday, 31 July 2026, after the test results were assessed. Later reporting identified the building as the headquarters of retailer Manor, although the health authorities continue to describe the cooling towers as the most likely source rather than a conclusively proven origin.
The affected systems are now undergoing remediation and currently pose no known public-health risk, according to the Basel-Stadt Health Department. Six other facilities exceeded the general Legionella limit, but testing has so far detected only Legionella species considered non-pathogenic to humans in those systems. Authorities have nevertheless ordered measures to prevent any possible emissions.
Prosecutors Open Criminal Proceedings
The death reported in Basel involved a 69-year-old man with several pre-existing conditions.
The Basel Public Prosecutor’s Office has opened criminal proceedings on suspicion of manslaughter in connection with the death. The proceedings were opened ex officio, based on the information available to authorities at the time, and the criminal investigation department is now examining the case.
Because the investigation is ongoing, prosecutors have not released further details about possible responsibility or whether any person or company is formally under investigation. The opening of proceedings does not establish criminal liability, and any responsibility will have to be determined through the legal process.
The case could focus on whether the cooling systems were properly designed, maintained and monitored, and whether operators complied with the relevant technical and hygiene requirements. The Swiss Association for Air and Water Hygiene has said that properly designed and maintained evaporative cooling systems do not pose a health risk, but stressed that responsibilities must be clearly assigned and staff must be qualified.
The Basel-Stadt Health Department said it would continue its inspections and testing until officials are satisfied that the public-health risk has been fully eliminated. Authorities have also called on operators of larger industrial facilities using wet cooling systems to contact the Cantonal Laboratory, as such installations are not all subject to mandatory reporting.
The latest figures indicate that the number of cases is still being updated as laboratory results and health reports become available. While the suspected source has been shut down, officials have not yet declared the investigation complete.
