In 2018, WHO formalised the term ‘Disease X’. Included in their list of priority diseases for research and development, Disease X represents the next unknown disease of epidemic potential. In Disease X: the 100 days mission to end pandemics, Kate Kelland explores how the world can possibly prioritise research and development on a disease that is not yet known, or perhaps not yet persisting in humans at all. An ambitious goal, but not unattainable.