Although the Wuhan coronavirus is currently dominating headlines across the globe, influenza kills hundreds of thousands of people worldwide each year. In the US, millions of people roll up their sleeves annually for a flu shot. But this ritual is confusing for many. Why is it that most vaccines are effective for a lifetime while the flu vaccine is only effective for a year? And why do we sometimes get the flu even when we’ve gotten the vaccine? The answer is evolution: the flu is constantly evolving to evade our immune systems. In this episode of Stereo Chemistry, scientists who study flu evolution and pandemics explain what makes fighting the flu so difficult. For more, visit cenm.ag/flucast. Image credit: US Army photographer