If you can get a little more specific about the type of fish you see this on, and what the hump looks like, the better.
Many fish, especially livebearers, wind up with a humped back, not unlike scoliosis in humans. Not cureable (well, they give humans a metal rod on the spine to straighten them up, I know, I nearly had to have one myself when I was a kid). But as long as the fish can swim, it's usually no big deal to the fish.
On the other hand, tetras (especially neons) can get what's called "neon tetra disease" which may form a whitish hump thingie on the back. I dunno if there is a cure for this, most books I've read recommend that you IMMEDIATELY remove the infected fish and euthanize it. This IS a fatal disease.