From Ephemeral New York:
North Brother Island is a 13-acre spit of land in the East River, between the Bronx and Riker’s Island. Unlike bigger Roosevelt Island nearby, it’s never been developed.
RiversidehospitalnobrotherBut it has been inhabited by people—sick people. Acquired by the city in 1885, officials built Riverside Hospital (at right) there, a place to quarantine New Yorkers who suffered from potentially deadly and easily communicable diseases such as typhus and smallpox. It also housed drug addicts until the 1960s.