Of all the coronavirus’s qualities, perhaps the most surprising has been that seemingly healthy people can spread it to others. This trait has made the virus difficult to contain, and continues to challenge efforts to identify and isolate infected people.

Scientists are beginning to untangle one of the most complex biological mysteries of the coronavirus pandemic: Why do some people get severely sick, whereas others quickly recover?

Since April, the Trump administration has funneled $48 million into a program with the Mayo Clinic, allowing more than 53,000 Covid-19 patients to get plasma infusions. Doctors and hospitals desperate to save the sickest patients have been eager to try a therapy that is safe and might work. Tens of thousands more people are now enrolled to get the treatment that’s been trumpeted by everyone from the president to the actor Dwayne Johnson, better known as The Rock.