+ - + - Estimated chance of epidemic 10 60 75 90% By The New York Times · Source: Emily Javan, Spencer Fox and Lauren Ancel Meyers, the University of Texas at Austin

As the coronavirus spreads silently through American cities and towns, people are struggling with questions about the benefits of social-distancing guidelines — especially in places that still have few reported cases.

Is the epidemic here yet? Is staying home and limiting contact with others really worth the trouble?

A new study by disease modelers at the University of Texas at Austin gives an answer: Even counties with just a single reported case have more than 50 percent likelihood that a sustained, undetected outbreak — an epidemic — is already taking place.

“I worry that many local officials are waiting until there is clear evidence of local transmission before taking action,” said Lauren Ancel Meyers, a professor of biology and statistics and one of the study’s authors. “The message is, we should not wait.”

With no consistent federal policy, local officials have been left on their own to decide when to enact control measures, Most should assume it is already here, she said.

Over all, the study finds, 70 percent of all counties in the United States — making up 94 percent of the country's population — are likely to have epidemics. The study defines an epidemic as an outbreak that grows exponentially instead of fizzling out on its own, eventually infecting a large fraction of the population.

Known cases in a county Probability of community transmission 0 9% 1 51% 2 70% 3 79% 4 84% 5 85% 10 95% 20 99% 43 or more 100% By The New York Times · Source: Emily Javan, Spencer Fox and Lauren Ancel Meyers, the University of Texas at Austin

The University of Texas research team simulated the spread of the outbreak in every United States county to determine the most likely outcome in each county, along with reported case counts compiled by The New York Times.

People who have been infected and have mild symptoms, or none at all, can pass the disease to others. Those cases, often undetected, are a prime driver of the outbreaks, Dr. Meyers said. Even in counties with no reported cases, there is roughly a 9 percent chance that an undetected outbreak is already underway, she said.

For those reasons, social distancing should be practiced across the United States, whether an outbreak now is visible or not, Dr. Meyers said.

“This virus spreads quickly and sometimes silently,” she said. “It's an unseen threat, and by the time you see it, it can be too late to intervene. You have to intervene proactively against threats you can't see.”

Social distancing can slow those outbreaks, giving local hospitals and other healthcare facilities time to handle the cases rather than being quickly overwhelmed, Dr. Meyers said.

“Most of us are probably living in communities where this virus is beginning to spread widely,” she said. “The prudent measures at this point are doing whatever possible to prevent an infected person from spreading the virus to an uninfected person. The common theme is do what you can to keep people from congregating.”