It is based on vaccinia, a close but harmless relative of smallpox virus.

Gene therapists have long tried to insert curative human genes into a patient's cells on the back of harmless viruses.

A few years ago, I was infected by a relatively harmless virus.

And in genetic medicine experiments, harmless viruses are used to carry desirable genes into cells.

However, it suppressed the immune system's ability to control normally harmless viruses, which led to fatal brain infections.

-Long ago, those of learning devised a virus, harmless to us, but deadly to Outworlders.

Then she mixes them with simple, harmless viruses whose genes have been reprogrammed to promote random variations.

She starts with a billion viruses, harmless to everything except bacteria, that have been genetically altered so they each create slightly different proteins.

Scientists have recently discovered a new and apparently harmless virus, HHV-7.

Eventually they'd 'infect the remaining population with the new, harmless virus.