I have a Nikon 35-70 F3.5 AI lens, which is pretty beat up, dented filter ring and all that, but optically sound.

Except, I've noticed a small amount of fungus at the back of the lens. It doesn't seem to affect my images at all, but what I want to know is: what's the worst that can happen if I ignore it? And would it be better to simply replace the lens rather than have it cleaned?