Disneyland has shut down two cooling towers after nine people who visited the California theme park came down with Legionnaires' disease.
The nine Disneyland cases are among 12 total cases of the bacterial lung infection that the Orange County Health Care Agency is investigating.
The patients, ranging in age from 52 to 94, lived in or had travelled to Anaheim, where the theme park is based, and nine had visited Disneyland in September.
One patient "with additional health issues", who had not visited the park, has died, health officials said.
There have been no new cases of the disease linked to Anaheim since September.
Disneyland was informed of the cases on 27 October.
Pamela Hymel, chief medical officer for Walt Disney Parks and Resorts, confirmed that two cooling towers had been shut down and treated with chemicals.
They will remain offline until tests confirm they are free from contamination.
The cooling towers provide cold water for various uses at Disneyland, such as refrigeration but not for drinking water, and give off a vapour or mist that could have carried the Legionnella bacteria.
Ms Hymel said that health officials had assured the company that there was no longer any risk to guests or employees of the park.
Legionnaires' disease can be spread through inhaling droplets from contaminated water sources.
While many people have no symptoms, it can cause serious pneumonia and prove dangerous to those with lung or immune system problems.
The disease, which is not contagious, can be treated with antibiotics.
Disneyland, California, which opened in 1955, attracts tens of thousands of visitors a day and is owned by The Walt Disney Company.