Seven months and more than a million deaths into the pandemic, scientists around the globe still don’t understand why some people infected with coronavirus get extremely ill and die, while others survive. The sheer range of outcomes for people who get COVID-19—from asymptomatic, to mild symptoms, to moderate disease leading to months-long complications, to death—has infectious disease doctors baffled. In a desperate race against time, researchers are rushing through work to better understand the disease, to find treatments and ultimately to develop an effective vaccine.