Seaview Hospital was opened in 1913 as a TB hospital. There were eight wards, 4 for men and 4 for women. It included the only treatments known at that time, fresh air, sleep, sun, healthy diet and a good view. When the hospital opened the New York Times called it “the largest and finest hospital ever built for the care and treatment of those who suffer from tuberculosis.” In 1943 a new medicine for tuberculosis were introduced patient rates drastically dropped. By 1961, Seaview was practically empty. Though some buildings lay abandoned, over 20 buildings on the campus are still used.
Me and https://abandonednyc.com/2015/05/20/the-sea-view-childrens-hospital/
I have been to Seaview three times and had no issues try to get in, but make sure to be careful because there is a lot of things still happening on the campus.