U.S. Rep. Mario Diaz-Balart, R-Miami, was the first member of Congress to test positive for COVID-19. Patients like him who survive the virus develop antibodies in their blood plasma. Doctors have been sharing survivors’ plasma with other patients who are fighting the virus, because it is believed to aid their immune systems. To collect plasma, a needle is inserted into a donor’s arm and a tube delivers the blood to a machine,...