In Germany, an HIV-positive man with leukemia needed a stem cell transplant—a treatment for some severe systemic cancers. His doctors wondered whether using stem cells from someone with resistance to HIV (people with two copies of a variant of the CCR5 gene produce HIV-resistant CD4 cells) might make the 40-year-old man HIV-free.

Not only did the transplant work—the stem cells took root, replacing the man’s immune cells—but his HIV went undetectable. More than a year later, it still doesn’t register in his system (in his tissue or blood). The experiment involved only one person, and stem cell transplants are highly toxic and costly (and require lifelong protective drugs). But it may suggest a new approach to an old hope: a cure. (Read more: “Transplanting Hope” on