11.   Then he took a normal version of the immune system gene the babies were missing, and added it to a harmless virus.

12.   They culled stem cells from the marrow and mixed them with a harmless virus in which a gene that makes the missing protein had been inserted.

13.   This is followed by an immunity booster, usually a harmless virus hollowed out to carry in more copies of the HIV genes.

14.   Patients in the new program will be injected with a harmless virus, stacked with a gene that will trigger tumour cells to make the potent drug interleukin.

15.   Some retroviruses are linked to cancer and the ability of the AIDS viruses to mutate could turn a seemingly harmless virus into a killer, he warned.