As Britain loses its measles-free status and cases surge worldwide, the World Health Organisation has labelled 'vaccine hesitancy" a top 10 threat to global health. But campaigners claim that vaccines are unsafe, and the cause of a health catastrophe. Decades of scientific research do not back them up. Yet their ideas not only persist - they are reaching around the globe. This documentary examines the arguments of those who campaign against vaccines, and hears from people at the heart of alleged conspiracies. And it analyses how vaccine-sceptic ideas are spreading, what gives them currency and who may be benefiting.