Photo: Mucosal barrier covering our epithelia functions as a protective shield against pathogen invasion in a way that is very similar to walls and a moat that were protecting medieval cities like Dubrovnik. Dehydration of this thick glycan layer inactivates this first line of defence and promotes both initial infection and subsequent expansion of the virus through airways (City of Dubrovnik, Diego Delso, delso.photo, License CC-BY-SA, via Wikimedia Commons).