The Covid-19 pandemic is the most significant health emergency of our time. Writing from Italy in lockdown, physicist and novelist Paolo Giordano explains how disease spreads in our interconnected world: why it matters; how it impacts us; how we must react. Expanding his focus to include other forms of contagion - from the environmental crisis to fake news and xenophobia - Giordano shows us not just how we got here but also how we can work together to create change.