Dr Kathleen O'Reilly, an epidemiologist at the London School of Hygiene and Tropical Medicine, joins us to talk about polio. The virus, long eradicated in many parts of the world, has been detected in the wastewater of several cities, suggesting it is now circulating. How did this happen, what are the risks, and what action needs to be taken?The Explainer is brought to you by The Journal. Providing open access to valuable journalism in Ireland has been the aim of The Journal for a decade. You can contribute to ensure we can keep questioning, investigating, debunking, explaining and informing at www.thejournal.ie/contribute/