A curious thing happened when Hong Kong reopened schools after closing them because of the Covid-19 pandemic. … Within a few weeks of schools reopening in October, they started to see large numbers of kids getting sick, despite mandatory mask-wearing, additional spacing between desks, and other measures to lower the risk of spread of SARS-CoV-2, the virus that causes Covid-19. …They were infected with rhinoviruses — one of the most common causes of the common cold.