With cases growing, city officials continue their investigation into identifying the source of the Legionnaires' outbreak on the Upper East Side. 

“We’re absolutely taking this seriously and launched an investigation the moment that the possibility of a cluster was on the radar," Dr. Alister Martin, the commissioner of the Department of Health and Mental Hygiene, said.

Officials believe the source of the bacteria is likely a cooling tower in the affected area. 

In May, a new law passed by the City Council went into effect that requires building owners to test their cooling towers more frequently. 

“That sampling window has been diminished to 31 days, and so there’s more routine testing,” Martin said.

“It's too early to see the impact of it but my hope, my expectation is [that] because of that narrower window of testing, we will be able to see more cases earlier on," he continued.

As of Monday, there have been at least 18 confirmed cases in three zip codes — 100128, 10028 and 10075 — on the Upper East Side with no reported deaths. 

The current outbreak comes less than a year after a deadly cluster impacted Harlem that sickened more than 100 people and killed seven. 

Legionnaires’ disease is a type of pneumonia caused by Legionella bacteria that thrive in warm water. Symptoms include cough, fever, muscle aches and difficulty breathing. 

“What we’ve been able to do is identify this region which has over 150 cooling towers and we’ve been hard at work with our environmental health inspectors pulling down those samples, sending those out to the lab and making sure we are serving remediation orders to the cooling tower owners,” the health commissioner said.

In a virtual town hall meeting Monday night, city officials said they had only tested one-third of the cooling towers in the affected areas. 

Council Speaker Julie Menin, who represents the district, was concerned about the sampling rate. She has been closely monitoring the outbreak and sharing information with residents. 

“When will they be sampled?” the speaker asked.

“Over the course of the weekend, we got about a third done. And over the course of the next several days, we’re going to get through the rest of the backlog,” Martin said.

 As for residents, they had mixed reactions to the concerning news.