Nuclear weapons are unique in their destructive capacity. A single nuclear weapon can devastate a city and kill hundreds of thousands of people in an instant.
A nuclear explosion means a severe humanitarian disaster. The intense heat of a detonation causes bodies to completely vaporize and cause severe burns. The pressure wave travels several miles crushing buildings, infrastructure and people. The radioactive fallout that follows can cause radiation sickness and cancer that can lead to death.
With hospitals in ruins, medical personnel killed or injured and a lack of medical equipment, it will be very difficult to treat the large number of the injured.