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?