I lost track of this short story, probably from the mid-to-late 1980's or early 1990's. The main character is an old "normal" woman, a scientist, who's always had to compete with genetically-enhanced people. Years after she retired from science work, one of the enhanced people that she used to work with comes to her for help with a sudden illness/genetic failure that's striking all the pretty people. It turns out that the "normal" woman caused it (or maybe just discovered it and didn't tell anyone?), years before, out of revenge.
I found it - it's Nancy Kress' Margin of Error, first published October 1994. Not quite as I remembered (I forgot about the adultery angle) but it's definitly the right story.
“You should have left David alone. You already had Zweigler; you should have left me David. Our marriage was never the same after that.”
She said, “I’m dying, Karen.”
I turned my head from the nursing babies to look at her.
“It’s true. My cellular machinery is running wild. Just in the last few months. The nanoassemblers are creating weird structures, destructive enzymes. For five years they replicated perfectly and now…For five years it all performed exactly as it was programmed to—”
I said, “It still is.”