Twelve cases of Legionnaires disease have been found in recent Disneyland visitors. The outbreak was traced to two cooling towers contaminated with the bacteria.

Legionnaires is a severe form of pneumonia, caused by exposure to contaminated water, or mist. It is contracting by inhaling the microscopic water droplets that contain the bacteria and develops two to ten days after exposure. Outbreaks have been linked to a variety of water sources: hot tubs, swimming pools, air conditioners, and other water systems in large buildings. It is treatable via antibiotics (while they still work anyway), but can lead to life-threatening complications and may be fatal. People with weakened immune systems are especially at risk.

Meanwhile, Disney has shut down the contaminated cooling towers and treated them with chemicals to destroy the bacteria. A bit of an ominous uptick in Legionnaires has been recorded this year. According to the LA Times:

Orange County has recorded more than 55 cases of the disease this year and has seen the number of cases jump in recent years. A similar upward trend has been seen nationally and elsewhere in Southern California, according to the healthcare agency, though what’s causing that is unclear.