Chimera: Human-Sheep

I'm worried about this for this reason, quoted at the bottom of the story:

But the development is likely to revive criticisms about scientists playing God, with the possibility of silent viruses, which are harmless in animals, being introduced into the human race.

Dr Patrick Dixon, an international lecturer on biological trends, warned: "Many silent viruses could create a biological nightmare in humans. Mutant animal viruses are a real threat, as we have seen with HIV."