Plenty of words begin with the prefix dis-, meaning 'not' or 'opposite' or 'negation' or something of that sort. Many examples will pop into your mind immediately.

But remember today's post about "erectile dysfunction, or ED"? Off the top of my head I can't think of any other example of the dys-form. Doubtless there are some, but surely nowhere near as many as dis-.

My first thought was that dysfunction was just a relatively rare form of disfunction. But it turns out that it's not rarer; it gets far more google hits.¹

So? Why the y here, and near-uniquely here?

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¹Even omiting d(i)(y)sfunctions of the 'erectile' or 'sexual' sort:
1,780,000 ghits for dysfunction -erectile -sexual
308,000 ghits for disfunction -erectile -sexual

Dys- is more common in medical words, like dysentery, dyscrasia, etc. I'd say it's similar to the Alzheimer's Disease discussion we have a while back.