Tuberculosis
Symptoms are Hollow belly, Knife back, Pale skin, Bending of spine.
They also lose their appetite which does not help them.
If suspected it is best to dispose of the fish immediately.
TB can be passed on to humans but only if you have open wounds on your hands when placing them in the aquarium.
Genetics
As with all other animals genetic defects can take all forms, Bent spines, No tail or fins Etc.
Vitamin Deficiency
This can also cause abnormalities similar to TB and Genetic
defects.
Lack of Calcium and Vitamin C are the two main culprits, resulting in bent spine and abnormally small fish.