genetically altered hiv virus to get into human immune systems to change those cells to target cancer fighters. they went back in and destroyed the cancer. dr. steven gruff is a pioneer. using hiv to infiltrate the immune system. >> all of the things that make the hiv virus to cause disease have been removed from this particular virus, so its only purpose is to put a gene into a cell. for me, it's incredibly exciting. >> reporter: here's emma, just eight months later. doctors can't find any remaining cancer cells. >> she is in complete remission. she has no leukemia in her body, by even the most sensitive tests. >> she has a ton of energy. she's doing wonderful right now. >> this treatment was a miracle. >> reporter: emma is counting the days until christmas. her parents are busy counting their blessings. the treatment does wipe out part of the immune system, so she has