I have 2 guppies with bent spines, had them since they were born in my tank. They just look different from the others but it hasn't affected them from reproducing and living healthy lives.
if it was a birth defect would it be fine and then one day, over night, have a severly curved spine? can it arise that quickly, or does it have to be born with it?
No disease could lead to such changes overnight unless it's something like dropsy. Changes in the spine that significant either occur slowly or from injury.