The unfortunate common thread between people experiencing an outbreak of a bacterial illness called Legionnaires' disease in Anaheim, California, is that nine of the 12 visited Disneyland in September, The Associated Press reports. The remaining three live in or traveled to Anaheim.

Here's what you need to know about the disease and the latest outbreak:

When were the cases discovered? The Orange County Health Care Agency said the cases of the bacterial illness were discovered about three weeks ago.

One patient, who hadn’t visited the park, has died.

The health agency said there haven’t been any new cases reported.

How did Disneyland respond? Disneyland said it learned about the Legionnaires' cases on Oct. 27 and shut down and disinfected two cooling towers that had high levels of the bacteria. The towers will reopen after it's confirmed they are no longer contaminated.

What is Legionnaires' disease? The Mayo Clinic describes the illness as "a severe form of pneumonia" caused by a bacterium known as legionella.

How did it get its name? Legionnaires' disease got its name in 1976 from an outbreak that caused 182 people attending an American Legion convention in Philadelphia to fall ill, Deadline reported. Twenty-nine people died.

“The outbreak was traced to the convention hotel’s air conditioning system, and Legionnaires’ disease has since been sourced to contaminated water or mist,” Deadline added.

What causes it and how does it spread? Legionnaires' is spread by mist from contaminated water. While Legionnaires' does not spread person to person, it does spread easily by inhalation.

The Mayo Clinic also lists the following common sources of outbreak:

  • Hot tubs and whirlpools on cruise ships
  • Grocery store mist machines
  • Cooling towers in air conditioning systems
  • Decorative fountains
  • Swimming pools
  • Physical therapy equipment
  • Water systems in hotels, hospitals and nursing homes

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– The Associated Press and the Cox Media Group National Content Desk contributed to this report.