She sets out to recruit the nation to download the BBC Pandemic app in a ground-breaking experiment to help plan for when the next deadly virus comes to the UK. How quickly will it spread? How many could it kill? What can we do about it? The BBC Four Pandemic experiment will find out.

Hannah masterminds the experiment and adopts the role of Patient Zero by walking the streets of Haslemere in Surrey to launch the outbreak. Meanwhile, emergency physician Dr Javid Abdelmoneim finds out why flu is still such a danger to society a century after Spanish flu killed up to 100 million people worldwide. He meets researchers trying to discover what makes some people more contagious than others 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, Hannah and Javid make a shocking revelation.