One person is dead and six other people have been sickened in an outbreak of Legionnaires’ disease on the Upper East Side of Manhattan, the city health department announced on Friday.

The patients with the bacterial infection, which is typically contracted through contaminated water, fell ill within the past 11 days in the Lenox Hill neighborhood, said the agency, the Department of Health and Mental Hygiene.

Four remain hospitalized, and two have recovered and been released. The person who died was over 90, according to the department, and suffered from other health problems. The department has begun an investigation of air-conditioning equipment in the neighborhood, looking for signs of the Legionella bacteria.

“We know that this is an organism that exists in our environment, and we don’t expect to be able to eradicate it,” said Dr. Mary T. Bassett, the health commissioner. “From a public health point of view, we want to be able to get a handle on clusters that may have a common source, but we hardly ever are able to identify them.”