Red Belly, or sores under the body, is an advanced bacterial infection wherein the bacteria have invaded the internal organs. This is very characteristic of Aeromonas infections. Injections are required to save such fish, and even then, it's too late for the really advanced cases. Injections all around, even healthy-exposed fish to save the ones who've not got too bad. 

Consult the professional Koi Dealers if you are unsure about administering these anti-biotic injections.