To cure a genetic disorder, scientists have proposed inserting a copy of the healthy gene into a virus. Then they would use the virus to infect the patient. Ideally, the healthy gene would transform the patient's genome. Scientists have conducted many trials of this procedure. Which is the MOST HARMFUL of the potential drawbacks of this procedure?

A. The patient's genome does not take in the copy of the healthy gene.
B. The virus does not infect the patient.
C. The patient's immune system successfully attacks the virus.
D. The viral genes are more harmful than expected.