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.

It begins with a mild fever and malaise, followed by a painful cough and shortness of breath. The infection prospers in crowds, spreading to people in close reach. Containing an outbreak requires contact tracing, as well as isolation and treatment of the sick for weeks or months.

This insidious disease has touched every part of the globe. It is tuberculosis, the biggest infectious-disease killer worldwide, claiming 1.5 million lives each year.

An experimental coronavirus vaccine developed by Johnson & Johnson protected monkeys from infection in a new study. It is the second vaccine candidate to show promising results in monkeys this week.

The company recently began a clinical trial in Europe and the United States to test its vaccine in people. It is one of more than 30 human trials for coronavirus vaccines underway across the world. But until these trials are complete — which will probably take several months — the monkey data offers the best clues to whether the vaccines will work.