With nation states across the world struggling to contain the coronavirus pandemic, there’s an urgent need for an internationally coordinated response. That’s where the UN agency the World Health Organisation should have a vital role to play; but right now, the WHO is at the centre of a political storm. Donald Trump has withdrawn US funding, accusing the agency of being China-centric. Stephen Sackur speaks to David Nabarro, the WHO special envoy for Covid-19. Is the organisation failing its greatest test?