Orange County health officials said nine people who visited the Anaheim theme park in September developed the disease.
An additional three people who had been to Anaheim but not Disneyland got sick, said Jessica Good, a spokeswoman for the Orange County Health Care Agency (OCHCA). One patient, who had not visited the park and had additional health issues, died, she said.
The 12 patients are between ages 52 and 94, and 10 were hospitalized, she said.
According to Dr. Pamela Hymel, chief medical officer for Walt Disney Parks and Resorts, the towers were shut down after Disney was contacted by the county health care agency on October 27 about increased Legionnaires' cases in Anaheim.
"We conducted a review and learned that two cooling towers had elevated levels of Legionella bacteria," Hymel said.
"These towers were treated with chemicals that destroy the bacteria and are currently shut down. We have proactively shared this information with OCHCA and given our actions, they have indicated there is no longer any known risk associated with our facilities."
Good also said there was "no known ongoing risk with this outbreak."
"To date, no additional Legionella cases have been identified with potential exposure in Anaheim after September," Good said.
Legionnaires disease is a severe form of pneumonia caused by Legionella bacteria, sometimes found in water systems. It is typically contracted by breathing mist from the water that contains it. The source of the mist can be air conditioning units in large facilities, showers or hot tubs. Legionnaires' disease is not contagious between humans.
County health officials identified Disneyland Park as a common location of eight of the cases last month, and have been working to identify potential sources, Good said.
Disneyland Park informed health officials this month that elevated levels of Legionella had been identified in two of its 18 towers, which were then treated and disinfected.
Disneyland took the towers out of service on November 1 and told the health agency it had performed additional disinfecting and testing. It brought the towers back into service November 5, but two days later, they were taken out of service again, she said.
Health officials subsequently issued an order that the towers remain shut down until they are verified to be free from contamination. The results of the tests will not be known for about two weeks.