When a serial killer is invisible, home is the best place to hide.

That is the logic behind the Bay Area’s dramatic new seven-county ‘shelter in place’ order, which aims to empty our streets, schools and stores. A virus dies when it can’t find someone new to infect.

“If this works, the San Francisco Bay Area will set an example,” said infectious disease expert Jeffrey Shaman of Columbia University’s Mailman School of Public Health, who with colleague Sen Pei tracked the epidemic’s rise and fall in China. “If it’s effective, others will follow.”

Unwittingly, we’ve been doing COVID-19’s bidding, according to an emerging body of research. The virus can live, virtually unnoticed, in young healthy people – then jump, without warning, into the weak, old and vulnerable.

That’s what worries Bay Area health officials: In an increasing number of cases, there’s no clue where the virus is hiding. It’s hit-and-run.

If the virus continues this spree, spreading mysteriously and exponentially, soon we could look like Seattle. Or, even worse, Italy.

Even people who aren’t sick with COVID-19 will feel its impact, say experts. Bay Area hospitals are already nearly full. With a surge of pneumonia cases, they could be too full to handle your shoulder surgery, diabetes care or chemotherapy. If there’s a major car accident, with multiple casualties? Good luck.

Similar control measures — but even stricter — are what helped flatten the trajectory of China’s devastating epidemic. China imposed travel restrictions between major cities, self-quarantining, rapid testing and “social distancing” on Jan. 23. Now their case count is falling.

A major new analysis, published on Monday by Shaman and Pei in the journal Science, reveals the central challenge of this new virus: People with mild or no symptoms are the main drivers behind its spread.

Six of every seven infections – 86% — were undetected in China before January’s strict travel restrictions, according to the team. The Columbia University team developed a mathematical model that simulated the dynamics of infection among 375 Chinese cities to estimate the contagiousness and proportion of so-called ‘undocumented’ infections during the weeks before and after the shutdown of travel in and out of Wuhan.

That time of unfettered travel and socializing in China was similar to life here in the busy Bay Area – before our new restrictions.

The problem, say experts, is “stealth transmission,” when people who are asymptomatic pass the virus along. Even if you feel fine, your virus could be fatal to someone else.

“We go about our day. We’ll still send the kids to school. We’ll still go to work. If we have a little bit of a headache or slight fever, we might take some Ibuprofen and still go out and go shopping,” said Shaman.

“You run into hundreds of people. And that allows the silent transmission of respiratory viruses,” he said. “It’s flying below the radar.”

“But if you’re on ‘house arrest,’ you reduce your contact to a handful of people, even fewer,” he said.

Fortunately, these hidden infections are only about half as contagious as symptomatic infections. That’s because people aren’t coughing and sneezing, spreading germs.

Yet they were found to be responsible for about two-thirds of China’s documented infections – because they are so abundant. These findings explain the rapid geographic spread of the disease and suggest that control could be difficult.

That’s why “social distancing” is so important.

“We need to keep people apart,” said Shaman.

If China’s numbers are applied to the U.S. population, this means that we have far more than the 1,629 infections currently reported by the U.S. Centers for Disease Control. The real number would be closer to 13,000.

Globally, this means nearly one million people are infected, according to the team.

The true number of infections will only be known with better and more routine testing, said Betz Halloran of the Fred Hutchinson Cancer Research Center in Seattle.

“It’s crucial to help slow the epidemic” because then more people can self-isolate, she said.

Meanwhile, other emerging research is revealing new insights about the disease:

The COVID-19 virus appears to be more easily transmitted than seasonal influenza. The “reproductive number” – the number of new infections generated from one infected individual – is estimated to be about 2.5 for this virus, which is higher than for the flu. In other words, each infected person spreads it to 2.5 other people. For an epidemic to vanish, transmission needs to drop below 1.

While the symptoms of flu and COVID-19 are similar, COVID-19 seems to be causing more severe disease. Of infections, 80% of infections are mild or asymptomatic, 15% are severe and require oxygen and 5% are critical, requiring ventilation. These fractions of severe and critical infection are higher than what is observed for influenza infection.

The time between cases in a chain of transmission is, on average, four days, according to a team of scientists from the University of Texas and France, China and Hong Kong.

Scientists still believe the “incubation period” between infection and eradication is between 2 to 14 days. Symptoms appear, on average, five days after infection. They start mild, then worsen.

Almost all viruses are seasonal, peaking in the winter and fading in the summer. Will transmission start to ease up? No one knows — but without marked changes in behavior, there are suspicions it will continue to creep along. “One might guess it will go down, but not go away, in summer months,” said Ira Longini Jr. of the University of Florida’s Center for Statistics and Quantitative Infectious Diseases.

Travel restrictions work, but only up to a point. They delay the spread of infection but don’t stop it. A recent study found that the travel quarantine of Wuhan delayed the overall epidemic progression into mainland China by only three to five days.