Basel legionellosis outbreak over: two cooling towers were the source

The legionellosis outbreak that began in Basel-Stadt at the end of July is over. The cantonal health department announced this on Thursday. No new cases have been recorded for more than a week. One person remains in hospital and does not require intensive care.

The overall toll of the outbreak is twenty-eight cases of illness and one death. The first public announcement was made on 3 August.

The two towers were the origin

The hypothesis put forward from the start, which pointed to two cooling towers as the origin of the infections, has now been confirmed scientifically. The cantonal laboratory worked with Basel University Hospital on DNA sequencing of the samples. Experts compared the genetic material of bacteria taken from the installations, since taken out of service, with that taken from infected patients.

The comparison established that the source identified was the correct one and that it has been eliminated. The authorities will nonetheless continue to analyse samples collected in patients' homes and will order further measures if necessary. So far no link has emerged between shower water in the homes examined and the outbreak.

Legionellosis is primarily a respiratory disease caused by bacteria of the genus Legionella, which occur naturally in water. These germs multiply above all at temperatures of roughly 25 to 45 degrees and where water stagnates for long periods in hot water systems or other water-carrying infrastructure. Infection occurs through inhaling very fine water droplets containing the bacteria, for example from a shower. There is no person-to-person transmission, and drinking contaminated water does not cause infection.

The Basel case fits a long-term trend. According to the Federal Office of Public Health, reports of legionellosis in Switzerland have been rising since 2001. In 2025 the office recorded 529 probable and confirmed cases.

Cooling towers are common in administrative and industrial buildings, and their number is likely to grow as summer temperatures rise. The Basel finding shows where inspections should be concentrated.