When dealing with L. pneumophila serogroup 1, the urine antigen test is useful for early detection of Legionnaires disease and initiation of treatment, and has been helpful in early detection of outbreaks. However, it does not identify the specific subtypes, so it cannot be used to match the person with the environmental source of infection. The Legionella bacteria can be cultured from sputum or other respiratory samples. Legionella spp. stain poorly with Gram stain, stain positive with silver, and are cultured on charcoal yeast extract with iron and cysteine (CYE agar). A significant under-reporting problem occurs with legionellosis. Even in countries with effective health services and readily available diagnostic testing, about 90% of cases of Legionnaires disease are missed. This is partly due to the disease being a relatively rare form of pneumonia, which many clinicians may not have encountered before, thus may misdiagnose. A further issue is that people with legionellosis can present with a wide range of symptoms, some of which (such as diarrhea) may distract clinicians from making a correct diagnosis. Prevention
Although the risk of Legionnaires disease being spread by large-scale water systems cannot be eliminated, it can be greatly reduced by writing and enforcing a highly detailed, systematic water safety plan appropriate for the specific  facility involved (office building, hospital, hotel, spa, cruise ship, etc.) Some of the elements that such a plan may include are:

Keep water temperature either above or below the 20–50 °C (68–122 °F) range in which the Legionella bacterium thrives.
The cooling towers from which the people were infected in the latter outbreak had been cleaned during the summer outbreak, raising concerns about how well the bacteria could be controlled.On 28 August 2015, an outbreak of Legionnaires disease was detected at San Quentin State Prison in Northern California; 81 people were sickened and the cause was sludge that had built up in cooling towers.Between June 2015, and January 2016, 87 cases of Legionnaires disease were reported by the Michigan Department of Health and Human Services for the city of Flint, Michigan, and surrounding areas. The outbreak may have been linked to the Flint water crisis, in which the citys water source was changed to a cheaper and inadequately treated source. Ten of those cases were fatal.In November 2017, an outbreak was detected at Hospital de São Francisco Xavier, Lisbon, Portugal, with up to 53 people being diagnosed with the disease and five of them dying from it.In Quincy, Illinois, at the Illinois Veterans Home, a 2015 outbreak of the disease killed 12 people and sickened more than 50 others. It was believed to be caused by infected water supply. Three more cases were identified by November 2017.In the autumn of 2017, 22 cases were reported in a Legionnaires disease outbreak at Disneyland in Anaheim, California. It was believed to have been caused by a cooling tower that releases mist for the comfort of visitors.