The state health department confirmed Sunday morning that the number of cases of Legionnaires' disease in upper Manhattan has grown to 27.

The cluster is affecting people in Hamilton Heights and Washington Heights. Three people are hospitalized, the health department said Sunday.

Officials confirmed 12 days ago that one person died from complications related to the disease.

Most of the people who have been infected are over the age of 50. The number of cases has steadily grown over the past few weeks, with health officials originally confirming at least eight cases on July 11.

City officials warn New Yorkers to be vigilant if they live in lower Washington Heights or northern Hamilton Heights, as someone may have been infected and has not felt symptoms yet because of the disease's two-week incubation period.

Health officials said they have inspected more than a dozen cooling towers and ordered several building owners to increase their use of biocides to kill the bacteria associated with the disease.

Legionnaires' is a form of pneumonia contracted by breathing in water droplets contaminated with the bacterium Legionella. Most cases can be traced to plumbing systems where conditions are favorable for Legionella growth.

Adults who experience flu-like symptoms, fever, cough, or difficulty breathing should seek medical attention immediately.

The disease is not contagious and is easily treatable when caught early.

The city sees an average of 200 to 500 Legionnaires' cases each year.

NYC Health + Hospitals officials said Saturday night that low levels of the Legionella bacterium were found in the water supply of Jacobi Medical Center in the Bronx, but health department officials said that isn't connected to the cluster in upper Manhattan.