A hospital tank is NOT the same. A hospital tank is one that does not contain any other fish. Period. Plants, maybe. No fish. Defeats the purpose, and exposes the betta to the disease.
Fungus are few and far between, IME. Bacterial infections, especially in tanks with too many fish, especially nippers, are very common. Antibiotics are largely okay with crustaceans, and should be labeled as safe, but tend to wipe out the beneficial bacteria in the tank as well as any 'bad' ones, resulting in ammonia spikes that further stress the fish.