A total of 522 Danish blood donors were followed during 2004–2005 to describe the seroepidemiology of Legionella infections in healthy individuals from a general population. Antibodies to Legionella spp. were measured by indirect immunofluorescence antibody test. The prevalence of Legionella antibodies (titre ⩾1:128) was 26·8% and remained fairly constant during the year of follow-up. However, 6·9% of the blood donors developed a fourfold or greater rise in antibody titres. A history of visits to Danish summer cottages was associated with both Legionella seropositivity (OR 1·53, 95% CI 1·02–2·30) and seroconversion (OR 2·66, 95% CI 1·21–5·83). There were no consistent associations between either levels of antibody titres or seroconversion and self-reported health symptoms, absence from work due to illness, or to any risk factors. We conclude that community-acquired Legionella infections are frequent; however, they rarely result in severe illness.
