Mirror-Image Life

I believe the short story is "The Moat" by Greg Egan - it's in his short story collection Axiomatic.

There's also Rifters' series by Peter Watts (great reads IMHO). With as little spoilers as possible: an ancient bacteria made of some sort of opposite-chirality proto-RNA is wrecking havoc across the planet, as no modern life form is prepared for it.

Not quite that plot, but Change Agent by Daniel Suarez is about genetically engineered humans and includes a “right-hand man” with that trait who was originally designed to withstand epidemics, etc., that might harm normal humans. Link: https://www.goodreads.com/en/book/show/31396262

PS: I enjoyed the book.

The sci-fi plot I’ve wanted to write combines this with the idea of a non-orientable universe. A space shuttle goes off in search of habitable planets and happens upon a planet that looks exactly like earth, but with the continents reversed. It is earth, but as the travelers have done a full loop around the non-orientable universe, they now have a mirrored frame of reference. So they have to learn to survive as aliens among humans, not being able to eat the same food despite knowing everyone.

(Of course, this reader would have to suspend disbelief regarding the fact that earth should be billions of years in the future when the travelers arrived, and that the travelers would most likely be composed of antimatter upon arrival due to CP symmetry)