The Delta variant of the coronavirus is threatening to put the world in an entirely new stage of the pandemic. The variant is spreading fast, particularly in places with low vaccination rates — it is thought to be around 50 percent more transmissible than previous versions. What can be done to stop Delta, and how will the variant hamper global efforts to return to normalcy? Guest: Carl Zimmer, a science writer and author of the “Matter” column for The New York Times.