As the number of people infected with a novel coronavirus known as COVID-19 has ticked up worldwide—totaling more than 80,000 as of Tuesday afternoon—the U.S. Centers for Disease Control and Prevention (CDC) has routinely emphasized that the risk to Americans remains relatively low.

But that tone changed somewhat during a call with reporters Tuesday, when Dr. Nancy Messonnier, Director of the CDC’s National Center for Immunization and Respiratory Diseases warned that community spread appears imminent, and disruption to U.S. communities could be significant.

“It is not a question of if coronavirus will spread through the United States,” Messonnier said, “but a question of when and how many people will have severe illness.”

So far, person-to-person spread of COVID-19 has been minimal in the U.S. Most confirmed cases have been among Americans who traveled to areas where COVID-19 is more widespread, and these people have been treated and isolated accordingly. That’s crucial to the containment of disease, as spread within communities greatly increases the total number of cases and the likelihood that transmission will multiply.

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Messonnier said the growing number of countries that have seen the virus pass from person to person in significant numbers is worrying, and something the U.S. should consider.

“I’m concerned about the situation. CDC is concerned about the situation,” Messonnier said, adding that now is the time for businesses, hospitals, schools and families to start preparing for a potential domestic outbreak.

On the call, Messonnier laid out steps communities and individuals may have to take if person-to-person spread picks up, including school and workplace closures; voluntary home quarantines; postponing or canceling mass gatherings; and implementing surface cleaning measures in schools, homes and public places. Depending on the extent to which these measures become necessary, schools and businesses may have to conduct classes and work over the phone or internet platforms, she said. Hospitals may have to ask patients to delay elective procedures and conduct other appointments using telemedicine.

While these steps have not yet been taken, even noting their possibility is a marked escalation from some of the CDC’s prior advice on coronavirus containment, which has mostly been limited to frequent hand-washing and covering coughs and sneezes.

“We recognize that implementing [measures] at this level will be disruptive to people’s day-to-day lives” if they become necessary, Messonnier said, noting possibilities such as lost income and the need for alternate childcare arrangements. “We want to prepare people for that possibility.”

At a Tuesday afternoon press conference, officials with the Health and Human Services Department (HHS) and Principal Deputy Director of the CDC Anne Schuchat, said Messonnier’s comments were intended only for the public to be aware of what could happen down the line.

“Dr. Messonnier’s comments were really to frame what might happen in the future,” Schuchat said. “It’s very important to say that our efforts at containment so far have worked, and the virus is actually contained here in the United States. We don’t want to delay thinking about other possibilities, and it was really an educational moment that she talked about.”

Messonnier also raised another concern on the call, noting that there could be shortages of supplies like protective gear for health care workers in the event of a pandemic. Overseas, many Asian countries affected by COVID-19 have run short of supplies like face masks.

At the Tuesday HHS press conference, Stephen Hahn, U.S. Food and Drug Administration (FDA) commissioner, told reporters the agency is monitoring for any possible shortages of medical products, including protective products like masks. Though there are no shortages in the U.S. for those products, he said the FDA will continue to monitor.

Despite the gravity of her advice, Messonnier emphasized that the CDC routinely plans for pandemic situations, in the hopes of over-preparing for a situation that turns out to be of no great risk. “I don’t think that preparedness will ever go to waste,” she told reporters.

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Write to Jamie Ducharme at jamie.ducharme@time.com and Jasmine Aguilera at jasmine.aguilera@time.com.