The government rates the global outbreak of a deadly flu virus as a major threat to the UK. It could happen at any time. To predict the impact of the next pandemic more accurately than ever before, new data is needed. Dr Hannah Fry is on the case.  She sets out to recruit the nation to download the BBC Pandemic app in a ground-breaking experiment to help plan for when that happens. How quickly will it spread? How many could it kill? What can we do about it?  Hannah masterminds the experiment and adopts the role of Patient Zero by walking the streets to launch the outbreak. Meanwhile, an emergency physician finds out why flu is still such a danger a century after flu killed up to 100mi people. He meets researchers trying to discover what makes some people more contagious and visits a factory that will produce vaccine when the next pandemic flu virus emerges.  Armed with the information he gathers and the results of the BBC Four Pandemic experiment, they make a shocking revelation.