...Mexico is battling one of the worst coronavirus outbreaks in the world, with more than 52,000 confirmed deaths, the third-highest toll of the pandemic. And its struggle has been made even harder by a pervasive phenomenon: a deeply rooted fear of hospitals.

The problem has long plagued nations overwhelmed by unfamiliar diseases. During the Ebola epidemic in 2014, many in Sierra Leone believed that hospitals had become hopeless death traps, leading sick people to stay home and inadvertently spread the disease to their families and neighbors.

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?