Nitpick: Viruses don't consume food for energy, they feed off of the energy metabolism of their host cell. CCR5 is a receptor that HIV binds to while entering human cells. If it is mutated in a certain rare way, HIV has a hard time entering cells, and it needs to be inside the cell to replicate. By swapping out this guy's immune system with one that has mutant CCR5, they've prevented HIV from replicating in his body.