As the coronavirus landed in the U.S. in January, scientists began whispering about an apparent difference from its notorious sibling, the virus that caused SARS: People infected with this one could easily infect others — even if they had no symptoms.

The first reports seemed questionable, and many infectious disease experts didn’t believe them. Two months later, the virus has swarmed across the United States, forcing tens of millions into self-isolation. And now some experts believe that asymptomatic transmis sion — the passing of a virus from an infected person who feels just fine to others — is driving the pandemic.

Concerns about people without symptoms infecting others were part of the reason county health officers across California, including most of the Bay Area, this week began advising everyone to wear face coverings in public, whether they feel sick or not. The Centers for Disease Control and Prevention issued a nationwide advisory on Friday.

That’s an abrupt turn from earlier messages from federal and state public health authorities, who had said face masks did not prevent spread of disease and should not be used. On Wednesday, the California Department of Public Health stopped just short of recommending all people wear face coverings, but acknowledged that there “may be a benefit to reducing asymptomatic transmission.”

But many infectious disease experts, including some who previously rejected masks for prevention, said they’ve come around.

“I’m changing my view and my actions, since anybody could have (the coronavirus), including me,” said Warner Greene, a senior investigator with the Gladstone Institutes. “I’ve only been to the market once (since sheltering in place), and I wore a mask. I’m protecting myself in case someone else is infected, but I’m also protecting them in case I’m asymptomatic.”

The CDC and local public health advisories recommend that people wear cloth face coverings, not surgical or N95 masks, which are in short supply and should be used only by health care workers.

Viruses that cause respiratory illness usually spread when people are actively ill — especially when they are coughing and sneezing, releasing virus-laden droplets into the air around them. People who are infected and not yet feeling sick, or who will never experience symptoms, generally are less likely to spread the virus to others.

That was largely the case with SARS, the respiratory syndrome also caused by a coronavirus that infected about 8,000 people worldwide in 2004. It was known to be highly infectious, but primarily when people were well into the illness and very sick with cough and fever.

That made it fairly easy to contain the spread of SARS, because people could be quarantined as soon as they had symptoms.

But with COVID-19, the disease caused by the new coronavirus, it’s now believed that some large number of people — anywhere from 20% to 50% — will never have symptoms. Even those who do can spread the infection before they feel sick.

That makes controlling the new coronavirus a far more difficult challenge — and that’s part of the reason why it’s raged more explosively around the world than SARS ever did, now infecting more than a million people.

“The asymptomatic part is what’s causing this thing to go on,” said Jay Levy, an infectious disease expert at UCSF. “When people are sick they know it, and they’re going to be in quarantine. But the asymptomatics have a lot of virus and they’re healthy, so they don’t know it.”

Scientists are still figuring out exactly how the coronavirus spreads from person to person. It clearly is transmitted through droplets that come from the nose and mouth. It’s widely believed there are two ways most people become infected: by standing too close to an infected person and breathing in their droplets, or by touching a contaminated surface and then touching their mouth or nose.

The new coronavirus appears to thrive in the nose and mouth, even in healthy feeling people who transmit it simply by speaking or singing, or even just breathing hard, said Robert Siegel, a Stanford infectious disease expert.

“What we’re learning is that things that actively propel droplets — singing, coughing — are effective at transmitting the disease. Anything that actively ejects it into the airspace,” Siegel said.

The first reports of asymptomatic transmission emerged in mid-January, but many scientists dismissed them as anecdotal and therefore possibly not true, or they assumed such cases were rare. SARS also could be spread by someone without symptoms, but not often.

As more countries were overrun with outbreaks, and more data was collected, it became apparent that some significant percentage of people infected with the virus never got sick.

The awareness of asymptomatic transmission made shelter-in-place orders that much more important, public health experts said. If there was no way to identify and separate infected people from the rest of the public, then everyone needed to stay apart.

“We still have no idea how many total people are infected — I wouldn’t be surprised to see 20% to 30% of the United States infected,” Greene said. “So you have an incredible number of asymptomatic individuals running around spreading the virus. Extreme social distancing — sheltering in place — seems to be helping, because it’s keeping us away from these asymptomatic individuals.”

Erin Allday is a San Francisco Chronicle staff writer. Email: eallday@sfchronicle.com Twitter: @ErinAllday