We know that masks are a core tool in fighting the spread of COVID-19. What was not known is the exact reason why. The reason is the route of infection. SARS-CoV-2 (the virus which causes COVID-19) first attacks the cells in the nasal passages, and from there works its way down into the lungs. Using a mask to prevent the initial deposit of viral material in the nasal passages is an essential first-line defense in preventing the spread: