Legionella pneumophila is a major cause of waterborne disease in the United States, but little is known about its prevalence in rural homes supplied by domestic well water. With a citizen science campaign involving 57 such homes in Illinois, conclusive results from 39 homes showed intact L. pneumophila in 31% and 28% of warm and hot water samples, respectively. We conducted a quantitative microbial risk assessment (QMRA) for L. pneumophila infection risk using L. pneumophila data from three studies (ours, another on …