The city Health Department now says it's investigating 11 cases of Legionnaires' disease in a section of Upper Manhattan.
At least eight people have been taken to the hospital for treatment.
A community meeting was held Thursday night keep residents of Hamilton Heights and Washington Heights informed about the disease.
City Councilman Mark Levine is urging people to get tested if they feel sick.
"It's so important that people who have flu-like symptoms get to a doctor. Just be safe. Don't tough this one out," Levine said.
Legionnaires' is caught by breathing in water vapor that's been contaminated with legionella bacteria.
It's not contagious, and it's treatable if caught early.
The Health Department says it is testing cooling towers in the affected area as it looks for the source of the disease.