UN Ebola chief raises 'nightmare' prospect that virus could mutate and become airborne if it is not quickly brought under control
- UN warns Ebola virus currently plaguing West Africa could become airborne
- The longer it moves between human hosts the greater possibility of mutation
- The risk grows the longer virus is living within the human 'melting pot'
- NGOs have said the Ebola virus is currently infecting five people every hour
- More than 3,300 people have died from Ebola since the outbreak first began
- Officials call for 1,000 new Sierra Leone isolation centres to contain virus