Legionnaires' is caused by the Legionella bacteria, which thrives in warm-water environments and is typically spread through the inhalation of aerosolized water droplets, such as mist from cooling towers, hot tubs, and decorative fountains. Legionnaires’ disease is not spread from person to person, and most healthy people who are exposed to the bacteria do not get sick, a press release from Iowa HHS said.

All available evidence indicates cooling towers are the most likely source of the Marshalltown outbreak, although laboratory testing has not confirmed a direct source, the press release said. Each business center with a cooling tower in the affected area has completed disinfection.

“We are encouraged to see fewer new cases reported recently,” said Dr. Robert Kruse, the state medical director. “Our teams remain engaged with building owners on disinfection and it will take additional time to know if this decline reflects successful remediation.”

People who are more likely to experience severe illness from Legionnaires’ disease include those 50 and older, those with weakened immune systems, those with chronic respiratory conditions, and smokers. Symptoms most associated with pneumonia and Legionnaires' disease include fever, cough, shortness of breath, muscle aches, headaches, chest pain, and gastrointestinal symptoms.

Iowa HHS recommended that business and property owners take the following steps to help mitigate the spread of Legionella:

• Establish, maintain, and review a water-management program.

• Disinfect cooling towers and water systems to prevent bacteria growth.

• Perform routine water system maintenance and maintain records and logs.

• Perform Legionella testing through an accredited laboratory, particularly following any remediation efforts.