For a spiky sphere just 120 nanometers wide, the coronavirus can be a remarkably cosmopolitan traveler.

Spewed from the nose or mouth, it can rocket across a room and splatter onto surfaces; it can waft into poorly ventilated spaces and linger in the air for hours. At its most intrepid, the virus can spread from a single individual to dozens of others, perhaps even a hundred or more at once, proliferating through packed crowds in what is called a superspreading event.

Eight months ago, the new coronavirus was unknown. But to some of our immune cells, the virus was already something of a familiar foe.

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.