Three more sites in the Bronx have tested positive for the bacteria that cause the potentially deadly Legionnaires' disease.

New York City health officials said Wednesday that a total of 20 sites have tested positive for the bacteria, including 14 in the South Bronx where most of the cases have been centered.

That cluster has killed 12 people and sickened more than 100 others, but city officials say they believe there have been no new diagnoses there since Aug. 3.

The disease is a form of pneumonia. It's caused by breathing in mist contaminated with the Legionella bacteria.

It's especially dangerous for the elderly and for people with underlying health issues.

It's not clear what triggered the outbreak, which began last month.