TB is short for Tuberculosis. There is a Genus of bacteria called Mycobacteria. It has numerous species and some of them affect people, birds, animals and fish. The TB that infects fish causes a granuloma (lump of bacterial cells) to grow inside the fish and this can put pressure on the spine or organs and cause the back to bend like the fish in the picture.
Other things can cause the spine to bend too, including:
soft water preventing the fish from getting enough calcium to develop a strong skeleton,
being gravid (pregnant) and having lots of babies developing inside the fish.
old age, and intestinal worms.