Chris had his blood retested four years after his lifesaving bone marrow transplant. The donor’s marrow had provided what was needed to cure him but had left a surprise: the DNA in Chris’ blood was that of his donor, not his own. It makes sense, really: the goal of the procedure was to replace the weakened blood with a donor’s healthy blood. Yet even swabs of Chris’ cheeks, lips, and tongue showed the donor’s DNA. In some ways, he’d become someone else—though he retained his own memories, outward appearance, and some of his original DNA.

Chris’ experience bears a striking resemblance…