Doctors sound alarm about patients in their 30s and 40s left debilitated or dead. Some didn’t even know they were infected. The man was among several recent stroke patients in their 30s to 40s who were all infected with the coronavirus. The median age for that type of severe stroke is 74. As Oxley, an interventional neurologist, began the procedure to remove the clot, he observed something he had never seen before. On the monitors, the brain typically shows up as a tangle of black squiggles — “like a can of spaghetti,” he said — that provide a map of...