Diabetic retinopathy

Diabetic retinopathy is the damage diabetes does to the tiniest blood vessels in the retina, the layer at the back of the eye that captures the image. Those vessels are so thin they're hard to picture: a web of microscopic hoses spread across the entire back of the eye. As diabetes wears them down, they stop holding in what they carry, so fluid seeps out into the surrounding tissue or blood leaks out directly.

There's a form of this disease in which something else happens too: new vessels start sprouting on top of the retina where they don't belong, badly built. That version is called proliferative diabetic retinopathy, because proliferate means exactly that, to multiply. Picture an old wall where, on top of the pipes that are already dripping, someone keeps tacking up new makeshift ones across the surface.