Three years ago, on March 11, 2020, the World Health Organization declared COVID-19 a pandemic. The virus had spread across the globe, and the WHO announcement triggered unprecedented measures – governments ordered people to quarantine, borders were closed and mandatory masking became commonplace. Today, science reporter Ari Daniel tells us about a new series from NPR: "Hidden Viruses: How Pandemics Really Begin." From bats in Bangladesh to dogs in Malaysia, NPR's Global Health and Development team traveled far and wide to learn how scientists are studying spillovers – when a disease crosses from animals to humans – and what makes some viruses more deadly than others.