This property makes viruses seem ideal as gene vectors. Researchers reasoned that they could remove those parts of the virus's genetic material that make the virus dangerous, and replace it with a human gene that would treat disease. The viruses would infect cells, but instead of injecting virus DNA, they would inject the human gene. Even though the modified virus can't cause disease directly, the body may still recognize it as a foe and attack it. A vigorous attack by the body's own immune system can be dangerous in itself.
Some genes are too large for a single virus to carry.
These gene vectors are very selective. They don't deliver to every cell. In some cases, the gene never arrives in the part of the cells where it can function. This method has its advantages, however. Some viruses inject their DNA only into certain types of cells.