Silent spread of Covid 19 keeps scientists grasping for clues.

India:-Covid-19 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.

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 will attend business meetings.


And outbreaks could begin anew.


The first hints


As early as January, there were signs people could harbor the virus without showing symptoms. A 10-year-old boy in China who traveled to Wuhan had no symptoms but tested positive, along with six others in his family who had coughs and fevers. More troubling was a report out of Germany: A business traveler from China spread the virus to colleagues in Munich, even though she appeared healthy.


Still, many scientists remained unconvinced.


The concept of people unwittingly spreading disease has never been an easy one to grasp, from the polio epidemic of mid-century America to the spread of HIV decades later.


At the turn of the 20th century, a seemingly healthy New York cook named Mary Mallon left a deadly trail of typhoid infections that captivated the public and led to her being forced into quarantine on an East River island. “Typhoid Mary” remains a haunting symbol of silent spread.


As Covid-19 emerged, health officials believed it would be like other coronaviruses and that people were most infectious when showing symptoms like cough and fever, with transmission rare otherwise.


“We were thinking this thing is going to look like SARS: a long incubation period and no transmission during the incubation period,” said Lauren Ancel Meyers, a disease modeler at the University of Texas at Austin.


Behind the scenes, scientists like Meyers were sharing their alarming finding with health officials. By scouring the websites of Chinese health departments, Meyers and her team found more than 50 cases between Jan. 21 and Feb. 8 where the person who brought the virus home didn’t develop symptoms until after infecting a family member.


“When we looked at the data, we said, ‘Oh no, this can’t be true,’” Meyers said. “It was shocking.”


Clues on a cruise ship


Rebecca Frasure, who contracted the virus while aboard the Diamond Princess cruise, sat in bed in Japan in late February, frustrated to be kept hospitalized even though she didn’t have any symptoms.


“I’m perfectly healthy except having this virus in my body,” Frasure said.


Without widespread and frequent testing, it’s impossible to know how many people without symptoms might carry it. The Diamond Princess, which idled in the Port of Yokohama, while the virus exploded onboard, enticed researchers.


After an ill passenger tested positive, only those with symptoms initial

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