An eighth person has died in New York City's largest known Legionnaires' disease outbreak, Mayor Bill de Blasio's office said Wednesday night.

At least 97 people have been sickened with the bacterial pneumonia since the first suspected case on July 10. The cases are concentrated in the South Bronx, and cooling towers atop buildings are thought to be the origin of the flare-ups.

On Tuesday, seven people had died and 86 had been sickened.

All eight fatalities were of older adults with pre-existing health conditions, such as lung infections or compromised immune systems, the mayor's office said.

The city health department said Tuesday that epidemiologists believe the outbreak has been contained.

The cooling towers -- which tend to be used more during the summer because of air conditioning -- have been decontaminated. So far, five towers have tested positive for the bacteria.

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De Blasio said earlier this week that he would introduce legislation to create a citywide registry of cooling towers and mandate an inspection schedule. Currently, the towers are largely unregulated. Because there is no registry, officials have had to rely on aerial photographs.

Legionnaires' disease, named after the first identified outbreak at a 1976 American Legion convention in Philadelphia, is a treatable type of pneumonia. It's transmitted through air conditioning, showers, baths, cooling towers or other water sources. It's not contagious, and the city says its drinking water is safe.

Symptoms include coughing, shortness of breath, high fevers, muscle aches and headaches.