COVID-19, caused by the new pandemic coronavirus, is strangely—and tragically—selective. Only some infected people get sick, and although most of the critically ill are elderly or have complicating problems such as heart disease, some killed by the disease are previously healthy and even relatively young. Jocelyn Kaiser writes in Science that researchers are now gearing up to scour the patients’ genomes for DNA variations that explain this mystery. The findings could be used to identify those most at risk of serious illness and those who might be protected, and they might also guide the search for new treatments.