New York City health officials say water vapor spread through the air from a building's cooling tower likely caused the deadlyin the Lenox Hill area on the east side of Manhattan.
At least one person has died and at least six others have been infected by the outbreak, which took place in the past two weeks.
Officials held an information session on Monday in response to the outbreak and City Health Commissioner Dr. Mary Bassett fielded questions from about 100 neighborhood residents, CBS New York reports.
"It's not spread by one person coughing, and another person getting it. It spreads through," she said. Inhaling mist contaminated with Legionella bacteria can cause the illness, a severe form of pneumonia that mostly affects older people and those with weakened immune systems.
So why are cooling towers believed to be the source? "The pattern in a geographic area over a fairly small window of time is most consistent with a cooling tower," Bassett said.
Officials say the Health Department has taken samples from all 116 cooling tower systems within a half-kilometer area and have stepped up inspections.
"I don't want anyone to think that there's any border over which water mist won't come," Bassett said.
She said no new cases have been reported since last Wednesday.