We call this Mouth Rot and Fin Rot. Ulcers may appear. They begin as reddish spots on the skin of the fish and will eventually turn into large holes. They turn inward and infect internal organs as well. Often hidden on the underbelly of the fish we do not see them until the ulcer is in a more developed and dangerous stage.
Septicemia can occur.
This is infection from within and can cause your fish to spiral instead of swim. Bacterial Dropsy is another example of bacteria attacking from within. The eyes of the fish bulge as does his body and the scales along his side may stick out like a pinecone. In extreme cases you can smell bacteria. It smells bad.