For the New York City cases, health officials suspect that the people became infected through contact with urine from rats, because most cases in New York City are linked with exposure to rats or rat-infested environments, the DOHMH said.
All three of those who were infected had severe symptoms including kidney and liver failure.
City officials say the cases, occurring in the past two months, are the first such concentrated cluster.
At 750 Grand Concourse, a Health Department notice warned tenants that a resident of the building was one of the three people diagnosed, CBS 2 reported.More news: California asks Trump for help with Oroville Dam spillway
A rare disease called leptospirosis has left one person dead, and two others fighting for their lives, in the Bronx.
The Health Department said there are typically one to three cases of leptospirosis every year in New York City.
Leptospirosis is caused by a spiral-shaped bacterium known as Leptospira, which can infect animals and people.
"This is actually in one of the department's 15 rat reservoir areas in the Bronx", Bassett said.
Between 2006 and 2016, there were only 26 cases of leptospirosis reported among New York City residents. The bacteria can enter the body through broken skin (such as a cut or scratch), or through the mucous membranes in the eyes, nose and mouth, the CDC said. While it is rarely spread through person-to-person contact, it can prove fatal if not immediately treated. City officials advise concerned residents to avoid contact with rats or places where rats may have urinated, wash hands thoroughly with soap and water, use a solution of one part bleach to 10 parts water to clean possibly affected areas and wear shoes and protective gear in rat-prone locations. Commons symptoms include fever, chills, headache, muscle aches, vomiting and diarrhea.