The Chicago Department of Public Health reported an increase in cases of Legionnaires’ disease in the city this month. In the first three weeks of July (1-21), 49 cases of Legionnaires’ disease were reported in Chicago, which is about a 3-fold increase in cases compared to the same period in 2020 (16 cases) and 2019 (13). Among the 49 people with Legionnaires’ disease to date in 2021, 15 (31%) were admitted to the ICU and two individuals died. CDPH issued an alert to local healthcare providers on July 9 reporting this increase in cases. A similar increase in illness has also been reported in Michigan. CDPH is investigating to find any common geographic or facility exposures. Legionnaires’ disease follows a seasonal pattern in Chicago, with an increased number of cases reported from June to October each year. Prolonged building shutdowns or periods of limited use, as seen with COVID-19, can allow Legionella bacteria to grow and multiply in water systems. Legionella bacteria are found naturally in freshwater lakes and streams but can also be found in man-made water systems. People at high risk should see a doctor as soon as possible if they are experiencing unusual respiratory symptoms. Wear a mask when visiting a healthcare provider as these symptoms are also seen with COVID-19.