A little more than three years ago a medical team from Berlin published the results of a unique experiment that astonished HIV researchers. The German group had taken bone marrow—the source of the body’s immune cells—from an anonymous donor whose genetic inheritance made him or her naturally resistant to HIV. Then the researchers transplanted the cells into a man with leukemia who had been HIV-positive for more than 10 years. Although treatment of the patient’s leukemia was the rationale for the bone marrow transplant therapy, the group also hoped that the transplant would provide enough HIV-resistant cells to control the man’s infection. The therapy exceeded the team’s expectations. Instead of just decreasing the amount of HIV in the patient’s blood, the transplant wiped out all detectable traces of the virus from his body, including in multiple tissues where it could have lain dormant. The German researchers were so surprised by the spectacularly positive results that they waited nearly two years before publishing their data.