For those of us in the northern hemisphere, flu season is quickly approaching. This raises an important question: what will it mean for Covid-19? Could hospitals be overloaded? Is co-infection likely and could it make symptoms worse? Or, will transmission of Sars-CoV-2 prevent the spread of seasonal influenza? In the first of two parts, Ian Sample addresses the question of flu and Covid-19 by investigating how different respiratory viruses interact. Speaking with Prof Pablo Murcia, Ian explores the interplay when viruses meet – both on a population level, and on the human scale. Help support our independent journalism at theguardian.com/sciencepod