Viral modification

Need to use a virus which replicates by inserting its genes into the host cell's genome. This virus has three genes - A, B and C. Gene A encodes a protein which allows this virus to insert itself into the host's genome. Genes B and C actually cause the disease this virus is associated with. Replace B and C with a beneficial gene. Thus, the modified virus could introduce your 'good gene' into the host cell's genome without causing any disease