Doctors say a gene therapy that makes use of the AIDS virus has given a working immune system to 48 babies and toddlers who were born without one. Results released Tuesday show that all but two of the 50 children who were given the therapy in a study now have healthy germ-fighting abilities. The one-time treatment involves using a disabled version of the AIDS virus to deliver a gene the kids lack to form an immune system. It’s too soon to know if this will be a cure for the so-called “bubble baby disease,” but the children are doing well so far.