Sickle cell disease, which causes Covid-like symptoms — clotting, strokes, and severe oxygen deprivation — is one of the medical conditions that the Centers for Disease Control and Prevention says puts people at higher risk for severe illness from the coronavirus. Now, a research team is trying to determine whether the several million people who merely carry one copy of the sickle cell mutation — but do not have the disease itself — could be more vulnerable to Covid-19, and whether that might be one reason the virus is disproportionately sickening and killing Black Americans.