From Ebola to swine flu to HIV/Aids, viruses borne by animals have caused some of the most devastating epidemics in history. What will come next? In Sierra Leone, Joe Shute joins the scientists working to find Disease X – a virus that is as yet undiscovered, but which could have the potential to ravage populations. Photographs by Simon Townsley

The caves surrounding the village of Gbentu in northern Sierra Leone are held sacred in local culture. Kings have been laid to rest here, and tributes stowed deep within the crevasses. For centuries, in times of death or crisis, tribal elders have tramped through the jungle to hold secret rituals in the darkness.

Tonight, however, it is a dozen or so Sierra Leonean scientists in blue scrubs and gumboots who trudge through the gloom. As we approach, a chorus of nervous-sounding coughs echoes down the line.