taking a monster to fight a monster

In a rare sucsess for gene therapyaltered HIV have been used to attack mice tumors, using HIV to attack cancer. I'm normally very unbeat on GM and the wonders of science, but if this is able to make me worried about its safety then this must really be an area to tread carefully. However as genetic traits that tend to give better HIV resistance are being identified it might not be as insane as it sounds, use one genetically engineered thign to kill your cancer, then a second one to stop the thing killing the cancer killing your entire immune system (which gets a battering in conventional chemo and radio therapies anyway).