Fears of a U.S. “twindemic” are growing as the nation deals with children’s hospitals crowded with cases of respiratory syncytial virus (RSV) just as the number of people with the flu also rise across the country. Experts have been talking about a “twindemic” in connection with a simultaneous surge in COVID-19 and flu cases. The worry was that winter waves of COVID would coincide with the regular flu season, and that the combination could pose a threat to health systems. Thankfully, COVID-19 cases and hospitalizations have leveled off in the United States — although that could change as temperatures drop...