We report about a 38-year-old female patient with an extremely severe case of legionellosis. The patient had to be treated in the intensive care unit for more than a month. Extracorporeal membrane oxygenation (ECMO) had to be established in order to save the patient's life. The patient did not belong to any risk group (i.e., age >50, smoker, immunosuppression, chronic disease, male sex). Infection control investigations carried out by the Public Health Department could not reveal concrete exposures likely to cause the infection. The serotypes in the patient material (Legionella pneumophila serotype 2-14) and in the samples taken from the showerhead in the patient's apartment (Legionella pneumophila serotype 1) were not identical. Results of the examination performed according to the German Drinking Water Ordinance (Trinkwasserverordnung, TrinkwV) carried out in 2017 in the patient's apartment, showed that the technical measures limit (Technischer Maßnahmenwert, TMW) was not exceeded. The patient has survived through the extensive use and efforts of intensive care measures. In general, a concrete exposure to Legionella is often not ascertainable, as in the case presented. This raises the question of how and whether such cases of legionellosis are preventable.