A sixth person has died in connection with a monthlong cluster of Legionnaires' disease on Manhattan's Upper East Side, New York City health officials said Tuesday, as the outbreak's total case count held at 90 amid signs the underlying source of contamination has been brought under control. At least 90 cases have been identified in the outbreak since it began in early July, according to the New York City Health Department, though no new cases have been diagnosed since July 17. "We are confident the source of the Legionella bacteria has been eliminated," the health department said in a statement Tuesday. Most patients recover from Legionnaires' disease with antibiotic treatment, but the illness can prove fatal in certain populations, including people who are immunocompromised, elderly, or living with chronic lung disease, according to the CDC. Nationally, Legionnaires' disease has become more common over the past decade.