Silent spread of virus keeps scientists grasping for clues

One of the great mysteries of the coronavirus is how quickly it rocketed around the world.

It first flared in central China and, within three months, was on every continent but Antarctica, shutting down daily life for millions. Behind the rapid spread was something that initially caught scientists off guard, baffled health authorities and undermined early containment efforts — the virus could be spread by seemingly healthy people.

As workers return to offices, children prepare to return to schools and those desperate for normalcy again visit malls and restaurants, the emerging science points to a menacing reality: If people who appear healthy can transmit the illness, it may be impossible to contain.

“It can be a killer and then 40 percent of people don’t even know they have it,” said Dr. Eric Topol, head of Scripps Research Translational Institute.

Researchers have exposed the frightening likelihood of silent spread of the virus by asymptomatic and presymptomatic carriers. But how major a role seemingly healthy people play in swelling the ranks of those infected remains unanswered — and at the top of the scientific agenda.

The small but mighty coronavirus can unlock a human cell, set up shop and mass produce tens of thousands of copies of itself in a single day. Virus levels skyrocket before the first cough, if one ever arrives. And astonishing to scientists, an estimated 4 in 10 infected people don’t ever have symptoms.

The slyness of the virus remains on the minds of many scientists as they watch societies reopen, wondering what happens if silent spreaders aren’t detected until it’s too late.

Travelers with no coughs can slip past airport screens. Workers without fevers won’t be caught by temperature checks. People who don’t feel tired and achy...