Legionnaires' outbreak in Basel: one dead, 26 infected, cooling tower suspected

Outbreak details

Over the past two weeks, 26 people in Basel have been diagnosed with Legionnaires' disease, a severe pneumonia caused by Legionella bacteria. One patient, a 69-year-old man with multiple pre-existing conditions, died from the infection. Of the 25 individuals who required hospitalisation, seven were admitted to intensive care. As of Monday, nine patients remain in hospital, and one is still in intensive care. The cases were concentrated in the Kleinbasel district, though a few isolated infections were detected in other parts of the city. Basel-Stadt normally records between 7 and 20 cases of legionellosis in a whole year, so 26 in a fortnight is far outside the canton's usual pattern.

“One person has unfortunately died, another is still in intensive care.”

Source and investigation

The clustering of cases in one neighbourhood prompted health authorities to search for a common source. Ten water-based cooling installations near the outbreak were tested. Eight exceeded the permitted Legionella threshold, and two of them contained Legionella pneumophila, the most dangerous species. The installation identified as the likely source, on the roof of an office building at Rebgasse in Kleinbasel, was shut down on Friday, 31 July. Cantonal chemist Yves Parrat said the other contaminated installations harboured only non-pathogenic Legionella species. The towers release water vapour that, depending on weather and wind, can spread over a wide area. The aerosols can be carried by wind and settle, entering homes through open windows or being inhaled on balconies and pavements.

“If residents inhale the water droplets, they can become infected.”

The most likely source, a cooling tower on a building roof, has been shut down. Basel-Stadt's health director, Lukas Engelberger, who also chairs the Swiss Conference of Cantonal Health Directors, gave that assessment at the press conference.

“We assume that the Legionella danger for the population has been eliminated.”

Disease and risk

Legionella bacteria multiply in warm, stagnant water at temperatures between 25 and 45 degrees Celsius. Infection occurs when people inhale fine aerosol droplets, for example from cooling towers, showers, or whirlpools. Drinking contaminated water is not dangerous because stomach acid kills the bacteria. The severe form, Legionnaires' disease, has an incubation period of two to ten days and leads to pneumonia with fever, cough, muscle pain, headaches, and sometimes confusion or gastrointestinal symptoms. Even with antibiotic treatment, 5 to 10 percent of patients die. A milder form, Pontiac fever, causes flu-like symptoms and resolves within days without treatment. Older adults, men, smokers, and people with weakened immune systems or chronic conditions such as diabetes, heart, or lung disease face higher risk.

Response and recommendations

The Basel-Stadt health department held a press conference on Monday afternoon. Deputy cantonal doctor Eva Würfel noted that because legionellosis is notifiable in Switzerland, authorities could rapidly trace the source. Officials urged anyone with pneumonia symptoms (shortness of breath, fever, severe cough) to see a doctor. The Federal Office of Public Health recommends keeping hot water at 60 degrees Celsius at the boiler outlet and 55 degrees in pipes, and cold water below 25 degrees, to prevent bacterial growth.

Broader context

Switzerland has recorded a steady increase in Legionnaires' disease cases. In 2025, 529 probable and confirmed cases were reported to the Federal Office of Public Health. Between 2000 and 2021, annual notifications rose fivefold from 140 to 677, before stabilising at around 650. The country has one of the highest per capita rates in Europe, for reasons that remain unexplained. In neighbouring Germany, more than 2,000 people contracted Legionnaires' disease in recent years, with 104 deaths in 2024 and 115 in 2023.